Refactors http_loader::load to be synchronous w/ an async wrapper
Refactors http_loader::load to be synchronous w/ an async wrapper

This simplifies the arguments that are passed in and should make testing
errors/responses easier once they're mocked

servo/servo#6727

mbrubeck/servo,brendandahl/servo,szeged/servo,eddyb/servo,caldwell/servo,DominoTree/servo,brendandahl/servo,splav/servo,huonw/servo,nnethercote/servo,dagnir/servo,CJ8664/servo,paulrouget/servo,DominoTree/servo,KiChjang/servo,caldwell/servo,emilio/servo,AnthonyBroadCrawford/servo,szeged/servo,mattnenterprise/servo,dsandeephegde/servo,GyrosOfWar/servo,WriterOfAlicrow/servo,brendandahl/servo,larsbergstrom/servo,paulrouget/servo,CJ8664/servo,upsuper/servo,sadmansk/servo,runarberg/servo,rnestler/servo,caldwell/servo,larsbergstrom/servo,eddyb/servo,dati91/servo,nrc/servo,peterjoel/servo,avadacatavra/servo,eddyb/servo,cbrewster/servo,dati91/servo,youprofit/servo,cbrewster/servo,anthgur/servo,AnthonyBroadCrawford/servo,peterjoel/servo,nrc/servo,szeged/servo,splav/servo,anthgur/servo,sadmansk/servo,nnethercote/servo,caldwell/servo,DominoTree/servo,caldwell/servo,aidanhs/servo,canaltinova/servo,sadmansk/servo,saneyuki/servo,cbrewster/servo,szeged/servo,avadacatavra/servo,bjwbell/servo,bjwbell/servo,larsbergstrom/servo,notriddle/servo,runarberg/servo,sadmansk/servo,bjwbell/servo,huonw/servo,upsuper/servo,emilio/servo,pyfisch/servo,saneyuki/servo,rnestler/servo,thiagopnts/servo,dsandeephegde/servo,ConnorGBrewster/servo,dagnir/servo,KiChjang/servo,j3parker/servo,anthgur/servo,splav/servo,jimberlage/servo,RenaudParis/servo,huonw/servo,CJ8664/servo,rnestler/servo,nnethercote/servo,fiji-flo/servo,pyfisch/servo,AnthonyBroadCrawford/servo,mbrubeck/servo,mbrubeck/servo,meh/servo,canaltinova/servo,upsuper/servo,peterjoel/servo,thiagopnts/servo,avadacatavra/servo,CJ8664/servo,steveklabnik/servo,huonw/servo,paulrouget/servo,mattnenterprise/servo,paulrouget/servo,sadmansk/servo,pyfisch/servo,aidanhs/servo,notriddle/servo,dsandeephegde/servo,thiagopnts/servo,dsandeephegde/servo,fiji-flo/servo,youprofit/servo,nnethercote/servo,saneyuki/servo,WriterOfAlicrow/servo,dagnir/servo,notriddle/servo,KiChjang/servo,upsuper/servo,splav/servo,anthgur/servo,RenaudParis/servo,cbrewster/servo,j3parker/servo,canaltinova/servo,mattnenterprise/servo,KiChjang/servo,notriddle/servo,nrc/servo,pyfisch/servo,dati91/servo,dati91/servo,brendandahl/servo,dagnir/servo,GyrosOfWar/servo,jdramani/servo,dati91/servo,dati91/servo,fiji-flo/servo,aidanhs/servo,mattnenterprise/servo,pyfisch/servo,jimberlage/servo,ConnorGBrewster/servo,aidanhs/servo,paulrouget/servo,AnthonyBroadCrawford/servo,pyfisch/servo,saneyuki/servo,upsuper/servo,dagnir/servo,larsbergstrom/servo,larsbergstrom/servo,splav/servo,paulrouget/servo,youprofit/servo,youprofit/servo,ConnorGBrewster/servo,WriterOfAlicrow/servo,KiChjang/servo,peterjoel/servo,szeged/servo,cbrewster/servo,GyrosOfWar/servo,ConnorGBrewster/servo,szeged/servo,j3parker/servo,SimonSapin/servo,DominoTree/servo,ConnorGBrewster/servo,KiChjang/servo,jimberlage/servo,fiji-flo/servo,meh/servo,splav/servo,dagnir/servo,eddyb/servo,jdramani/servo,emilio/servo,jdramani/servo,sadmansk/servo,meh/servo,SimonSapin/servo,brendandahl/servo,jimberlage/servo,paulrouget/servo,larsbergstrom/servo,peterjoel/servo,AnthonyBroadCrawford/servo,dsandeephegde/servo,notriddle/servo,upsuper/servo,caldwell/servo,anthgur/servo,thiagopnts/servo,RenaudParis/servo,saneyuki/servo,rnestler/servo,jdramani/servo,RenaudParis/servo,upsuper/servo,GyrosOfWar/servo,paulrouget/servo,RenaudParis/servo,avadacatavra/servo,emilio/servo,mbrubeck/servo,DominoTree/servo,SimonSapin/servo,meh/servo,nnethercote/servo,bjwbell/servo,aidanhs/servo,huonw/servo,splav/servo,SimonSapin/servo,avadacatavra/servo,dsandeephegde/servo,fiji-flo/servo,cbrewster/servo,larsbergstrom/servo,meh/servo,rnestler/servo,emilio/servo,RenaudParis/servo,WriterOfAlicrow/servo,anthgur/servo,ConnorGBrewster/servo,rnestler/servo,cbrewster/servo,DominoTree/servo,notriddle/servo,KiChjang/servo,ConnorGBrewster/servo,RenaudParis/servo,saneyuki/servo,youprofit/servo,larsbergstrom/servo,nnethercote/servo,steveklabnik/servo,brendandahl/servo,caldwell/servo,nnethercote/servo,runarberg/servo,runarberg/servo,WriterOfAlicrow/servo,SimonSapin/servo,saneyuki/servo,pyfisch/servo,eddyb/servo,emilio/servo,larsbergstrom/servo,nrc/servo,saneyuki/servo,SimonSapin/servo,steveklabnik/servo,anthgur/servo,jimberlage/servo,mbrubeck/servo,mattnenterprise/servo,CJ8664/servo,peterjoel/servo,eddyb/servo,ConnorGBrewster/servo,jimberlage/servo,avadacatavra/servo,meh/servo,thiagopnts/servo,nnethercote/servo,thiagopnts/servo,splav/servo,saneyuki/servo,SimonSapin/servo,mbrubeck/servo,WriterOfAlicrow/servo,WriterOfAlicrow/servo,bjwbell/servo,DominoTree/servo,emilio/servo,bjwbell/servo,dati91/servo,fiji-flo/servo,anthgur/servo,cbrewster/servo,GyrosOfWar/servo,steveklabnik/servo,peterjoel/servo,canaltinova/servo,rnestler/servo,splav/servo,runarberg/servo,notriddle/servo,j3parker/servo,GyrosOfWar/servo,youprofit/servo,eddyb/servo,dsandeephegde/servo,canaltinova/servo,DominoTree/servo,szeged/servo,mbrubeck/servo,jimberlage/servo,nrc/servo,CJ8664/servo,huonw/servo,pyfisch/servo,dsandeephegde/servo,jdramani/servo,steveklabnik/servo,splav/servo,pyfisch/servo,aidanhs/servo,thiagopnts/servo,sadmansk/servo,runarberg/servo,GyrosOfWar/servo,sadmansk/servo,szeged/servo,thiagopnts/servo,avadacatavra/servo,saneyuki/servo,brendandahl/servo,nnethercote/servo,jimberlage/servo,CJ8664/servo,canaltinova/servo,szeged/servo,nrc/servo,peterjoel/servo,mattnenterprise/servo,notriddle/servo,KiChjang/servo,szeged/servo,upsuper/servo,runarberg/servo,jdramani/servo,aidanhs/servo,KiChjang/servo,nnethercote/servo,rnestler/servo,AnthonyBroadCrawford/servo,j3parker/servo,j3parker/servo,SimonSapin/servo,youprofit/servo,fiji-flo/servo,emilio/servo,j3parker/servo,meh/servo,emilio/servo,mbrubeck/servo,pyfisch/servo,eddyb/servo,larsbergstrom/servo,AnthonyBroadCrawford/servo,notriddle/servo,peterjoel/servo,jimberlage/servo,CJ8664/servo,DominoTree/servo,avadacatavra/servo,fiji-flo/servo,KiChjang/servo,emilio/servo,steveklabnik/servo,peterjoel/servo,canaltinova/servo,jdramani/servo,dagnir/servo,mattnenterprise/servo,paulrouget/servo,mattnenterprise/servo,paulrouget/servo,nrc/servo,canaltinova/servo,bjwbell/servo,notriddle/servo,huonw/servo,steveklabnik/servo,dati91/servo,DominoTree/servo