Auto merge of #12275 - emilio:prefs-rwlock, r=jdm
Auto merge of #12275 - emilio:prefs-rwlock, r=jdm

prefs: Make the preferences be guarded by a RwLock instead of a Mutex

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This should reduce unnecessary locks that happen when we read preferences.

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larsbergstrom/servo,mbrubeck/servo,ConnorGBrewster/servo,paulrouget/servo,mattnenterprise/servo,notriddle/servo,pyfisch/servo,sadmansk/servo,eddyb/servo,notriddle/servo,canaltinova/servo,pyfisch/servo,nrc/servo,mattnenterprise/servo,mattnenterprise/servo,dsandeephegde/servo,emilio/servo,notriddle/servo,fiji-flo/servo,DominoTree/servo,mbrubeck/servo,paulrouget/servo,SimonSapin/servo,saneyuki/servo,rnestler/servo,thiagopnts/servo,cbrewster/servo,fiji-flo/servo,SimonSapin/servo,KiChjang/servo,peterjoel/servo,notriddle/servo,larsbergstrom/servo,mattnenterprise/servo,jimberlage/servo,KiChjang/servo,peterjoel/servo,larsbergstrom/servo,jimberlage/servo,paulrouget/servo,szeged/servo,DominoTree/servo,mattnenterprise/servo,cbrewster/servo,jimberlage/servo,SimonSapin/servo,saneyuki/servo,peterjoel/servo,szeged/servo,splav/servo,eddyb/servo,saneyuki/servo,ConnorGBrewster/servo,saneyuki/servo,splav/servo,CJ8664/servo,ConnorGBrewster/servo,SimonSapin/servo,sadmansk/servo,canaltinova/servo,upsuper/servo,mbrubeck/servo,sadmansk/servo,sadmansk/servo,DominoTree/servo,splav/servo,fiji-flo/servo,peterjoel/servo,pyfisch/servo,nrc/servo,rnestler/servo,peterjoel/servo,SimonSapin/servo,cbrewster/servo,sadmansk/servo,saneyuki/servo,mbrubeck/servo,thiagopnts/servo,mbrubeck/servo,dsandeephegde/servo,CJ8664/servo,splav/servo,DominoTree/servo,canaltinova/servo,rnestler/servo,canaltinova/servo,SimonSapin/servo,CJ8664/servo,canaltinova/servo,cbrewster/servo,emilio/servo,rnestler/servo,splav/servo,jimberlage/servo,notriddle/servo,thiagopnts/servo,emilio/servo,emilio/servo,DominoTree/servo,nnethercote/servo,avadacatavra/servo,pyfisch/servo,pyfisch/servo,avadacatavra/servo,anthgur/servo,notriddle/servo,szeged/servo,anthgur/servo,anthgur/servo,mattnenterprise/servo,nrc/servo,rnestler/servo,jimberlage/servo,cbrewster/servo,nnethercote/servo,CJ8664/servo,dsandeephegde/servo,canaltinova/servo,paulrouget/servo,KiChjang/servo,pyfisch/servo,anthgur/servo,paulrouget/servo,ConnorGBrewster/servo,rnestler/servo,thiagopnts/servo,upsuper/servo,dati91/servo,emilio/servo,dsandeephegde/servo,anthgur/servo,nnethercote/servo,upsuper/servo,CJ8664/servo,DominoTree/servo,jimberlage/servo,peterjoel/servo,anthgur/servo,pyfisch/servo,avadacatavra/servo,avadacatavra/servo,thiagopnts/servo,cbrewster/servo,rnestler/servo,dati91/servo,szeged/servo,upsuper/servo,thiagopnts/servo,ConnorGBrewster/servo,mattnenterprise/servo,eddyb/servo,paulrouget/servo,sadmansk/servo,pyfisch/servo,KiChjang/servo,emilio/servo,upsuper/servo,emilio/servo,jimberlage/servo,peterjoel/servo,anthgur/servo,upsuper/servo,saneyuki/servo,pyfisch/servo,larsbergstrom/servo,larsbergstrom/servo,SimonSapin/servo,nnethercote/servo,peterjoel/servo,jimberlage/servo,paulrouget/servo,saneyuki/servo,notriddle/servo,avadacatavra/servo,KiChjang/servo,splav/servo,upsuper/servo,sadmansk/servo,nnethercote/servo,SimonSapin/servo,dati91/servo,DominoTree/servo,fiji-flo/servo,thiagopnts/servo,nrc/servo,ConnorGBrewster/servo,nnethercote/servo,splav/servo,nrc/servo,larsbergstrom/servo,emilio/servo,splav/servo,sadmansk/servo,szeged/servo,szeged/servo,larsbergstrom/servo,CJ8664/servo,fiji-flo/servo,larsbergstrom/servo,rnestler/servo,szeged/servo,dati91/servo,CJ8664/servo,nnethercote/servo,avadacatavra/servo,emilio/servo,dsandeephegde/servo,dati91/servo,saneyuki/servo,eddyb/servo,DominoTree/servo,eddyb/servo,larsbergstrom/servo,nnethercote/servo,cbrewster/servo,CJ8664/servo,splav/servo,dsandeephegde/servo,cbrewster/servo,dati91/servo,jimberlage/servo,nrc/servo,KiChjang/servo,avadacatavra/servo,larsbergstrom/servo,fiji-flo/servo,canaltinova/servo,mbrubeck/servo,notriddle/servo,thiagopnts/servo,emilio/servo,mbrubeck/servo,canaltinova/servo,DominoTree/servo,splav/servo,dati91/servo,notriddle/servo,szeged/servo,ConnorGBrewster/servo,mbrubeck/servo,saneyuki/servo,eddyb/servo,avadacatavra/servo,dsandeephegde/servo,nnethercote/servo,fiji-flo/servo,KiChjang/servo,dati91/servo,fiji-flo/servo,nnethercote/servo,eddyb/servo,saneyuki/servo,dsandeephegde/servo,szeged/servo,mattnenterprise/servo,KiChjang/servo,pyfisch/servo,KiChjang/servo,ConnorGBrewster/servo,peterjoel/servo,nrc/servo,KiChjang/servo,paulrouget/servo,peterjoel/servo,anthgur/servo,szeged/servo,notriddle/servo,upsuper/servo,eddyb/servo,paulrouget/servo,paulrouget/servo,DominoTree/servo