Better locking within handle_box_url
Better locking within handle_box_url

This improves locking in the face of parallel providers and
handling box_url parameters. This avoids downloading a box multiple
times.

philoserf/vagrant,mitchellh/vagrant,aneeshusa/vagrant,kalabiyau/vagrant,bryson/vagrant,taliesins/vagrant,fnewberg/vagrant,apertoso/vagrant,cgvarela/vagrant,p0deje/vagrant,Endika/vagrant,kalabiyau/vagrant,tschortsch/vagrant,denisbr/vagrant,genome21/vagrant,wkolean/vagrant,Sgoettschkes/vagrant,channui/vagrant,darkn3rd/vagrant,mwarren/vagrant,darkn3rd/vagrant,patrys/vagrant,tbriggs-curse/vagrant,bdwyertech/vagrant,cgvarela/vagrant,nickryand/vagrant,gpkfr/vagrant,benizi/vagrant,justincampbell/vagrant,myrjola/vagrant,gajdaw/vagrant,rivy/vagrant,kamazee/vagrant,johntron/vagrant,juiceinc/vagrant,tjanez/vagrant,Endika/vagrant,juiceinc/vagrant,sni/vagrant,tomfanning/vagrant,aaam/vagrant,theist/vagrant,johntron/vagrant,invernizzi-at-google/vagrant,krig/vagrant,invernizzi-at-google/vagrant,pwnall/vagrant,tbarrongh/vagrant,philwrenn/vagrant,legal90/vagrant,carlosefr/vagrant,signed8bit/vagrant,ferventcoder/vagrant,Avira/vagrant,doy/vagrant,aaam/vagrant,h4ck3rm1k3/vagrant,jberends/vagrant,PatrickLang/vagrant,nickryand/vagrant,stephancom/vagrant,darkn3rd/vagrant,fnewberg/vagrant,signed8bit/vagrant,zsjohny/vagrant,bshurts/vagrant,lonniev/vagrant,MiLk/vagrant,gbarberi/vagrant,stephancom/vagrant,TheBigBear/vagrant,obnoxxx/vagrant,chrisvire/vagrant,mwarren/vagrant,philwrenn/vagrant,sax/vagrant,bheuvel/vagrant,p0deje/vagrant,teotihuacanada/vagrant,jean/vagrant,philwrenn/vagrant,mpoeter/vagrant,chrisvire/vagrant,miguel250/vagrant,tomfanning/vagrant,kamazee/vagrant,muhanadra/vagrant,loren-osborn/vagrant,dhoer/vagrant,sni/vagrant,dhoer/vagrant,teotihuacanada/vagrant,denisbr/vagrant,vamegh/vagrant,Chhunlong/vagrant,tschortsch/vagrant,gitebra/vagrant,aaam/vagrant,senglin/vagrant,Avira/vagrant,mpoeter/vagrant,jberends/vagrant,bdwyertech/vagrant,genome21/vagrant,jtopper/vagrant,kamigerami/vagrant,lonniev/vagrant,jberends/vagrant,philwrenn/vagrant,obnoxxx/vagrant,iNecas/vagrant,jfchevrette/vagrant,krig/vagrant,tbarrongh/vagrant,jfchevrette/vagrant,ianmiell/vagrant,benh57/vagrant,bshurts/vagrant,dhoer/vagrant,benizi/vagrant,loren-osborn/vagrant,tomfanning/vagrant,jmanero/vagrant,teotihuacanada/vagrant,dharmab/vagrant,sferik/vagrant,pwnall/vagrant,petems/vagrant,gpkfr/vagrant,senglin/vagrant,MiLk/vagrant,gajdaw/vagrant,stephancom/vagrant,crashlytics/vagrant,teotihuacanada/vagrant,rivy/vagrant,aneeshusa/vagrant,bdwyertech/vagrant,TheBigBear/vagrant,dustymabe/vagrant,chrisroberts/vagrant,mkuzmin/vagrant,marxarelli/vagrant,tbriggs-curse/vagrant,cgvarela/vagrant,dharmab/vagrant,clinstid/vagrant,shtouff/vagrant,tjanez/vagrant,blueyed/vagrant,juiceinc/vagrant,PatOShea/vagrant,nickryand/vagrant,lukebakken/vagrant,jmanero/vagrant,otagi/vagrant,channui/vagrant,gbarberi/vagrant,vamegh/vagrant,petems/vagrant,gitebra/vagrant,mitchellh/vagrant,tbarrongh/vagrant,bryson/vagrant,vamegh/vagrant,ArloL/vagrant,myrjola/vagrant,h4ck3rm1k3/vagrant,obnoxxx/vagrant,BlakeMesdag/vagrant,dustymabe/vagrant,juiceinc/vagrant,wangfakang/vagrant,kalabiyau/vagrant,Chhunlong/vagrant,marxarelli/vagrant,jean/vagrant,signed8bit/vagrant,doy/vagrant,cgvarela/vagrant,otagi/vagrant,gitebra/vagrant,johntron/vagrant,PatOShea/vagrant,benizi/vagrant,myrjola/vagrant,carlosefr/vagrant,wangfakang/vagrant,patrys/vagrant,ferventcoder/vagrant,jhoblitt/vagrant,clinstid/vagrant,sideci-sample/sideci-sample-vagrant,philoserf/vagrant,modulexcite/vagrant,vamegh/vagrant,muhanadra/vagrant,mwarren/vagrant,taliesins/vagrant,tbriggs-curse/vagrant,petems/vagrant,modulexcite/vagrant,legal90/vagrant,petems/vagrant,chrisroberts/vagrant,benizi/vagrant,dustymabe/vagrant,invernizzi-at-google/vagrant,nickryand/vagrant,mitchellh/vagrant,mpoeter/vagrant,doy/vagrant,shtouff/vagrant,legal90/vagrant,mwrock/vagrant,fnewberg/vagrant,myrjola/vagrant,taliesins/vagrant,dhoer/vagrant,mkuzmin/vagrant,benh57/vagrant,sax/vagrant,sideci-sample/sideci-sample-vagrant,lonniev/vagrant,mwrock/vagrant,signed8bit/vagrant,ArloL/vagrant,BlakeMesdag/vagrant,shtouff/vagrant,jkburges/vagrant,ianmiell/vagrant,bshurts/vagrant,modulexcite/vagrant,loren-osborn/vagrant,wkolean/vagrant,kamigerami/vagrant,Endika/vagrant,patrys/vagrant,jkburges/vagrant,theist/vagrant,bheuvel/vagrant,bheuvel/vagrant,kamazee/vagrant,patrys/vagrant,jtopper/vagrant,bheuvel/vagrant,otagi/vagrant,mpoeter/vagrant,Chhunlong/vagrant,jkburges/vagrant,Chhed13/vagrant,jhoblitt/vagrant,gitebra/vagrant,invernizzi-at-google/vagrant,bdwyertech/vagrant,Sgoettschkes/vagrant,gbarberi/vagrant,Avira/vagrant,janek-warchol/vagrant,benh57/vagrant,tjanez/vagrant,ArloL/vagrant,jhoblitt/vagrant,zsjohny/vagrant,wangfakang/vagrant,sferik/vagrant,kamazee/vagrant,aaam/vagrant,justincampbell/vagrant,h4ck3rm1k3/vagrant,jean/vagrant,jtopper/vagrant,PatOShea/vagrant,chrisroberts/vagrant,ianmiell/vagrant,Sgoettschkes/vagrant,h4ck3rm1k3/vagrant,carlosefr/vagrant,gajdaw/vagrant,aneeshusa/vagrant,blueyed/vagrant,jtopper/vagrant,channui/vagrant,jmanero/vagrant,dharmab/vagrant,shtouff/vagrant,lukebakken/vagrant,tschortsch/vagrant,Ninir/vagrant,Sgoettschkes/vagrant,kalabiyau/vagrant,mwrock/vagrant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