container merge_items speedup
container merge_items speedup

I rewrote ContainerBroker.merge_items to use bulk SQL operations.  This
performs identical to the current version when len(item_list) == 1.  But as
item_list grows, the new version performs better, up to around 50% faster when
item_list numbers in the hundreds.

I wrote a quick script to stress test merge_items:
https://gist.github.com/redbo/12a4ed9c6a020aee39fe

Average of several runs against master: 35.08 seconds
Average of several runs against this branch: 18.44 seconds

Change-Id: I07fff1fb05c90f1411c79d961e0ed63264ed2acf

iostackproject/IO-Bandwidth-Differentiation,dencaval/swift,anishnarang/gswift,daasbank/swift,smerritt/swift,anishnarang/gswift,nadeemsyed/swift,bkolli/swift,nadeemsyed/swift,revoer/keystone-8.0.0,tipabu/swift,mjzmjz/swift,clayg/swift,iostackproject/IO-Bandwidth-Differentiation,maginatics/swift,hbhdytf/mac2,Em-Pan/swift,dpgoetz/swift,aerwin3/swift,gold3bear/swift,levythu/swift,notmyname/swift,clayg/swift,williamthegrey/swift,redhat-openstack/swift,dencaval/swift,takeshineshiro/swift,gold3bear/swift,bouncestorage/swift,hurricanerix/swift,sarvesh-ranjan/swift,clayg/swift,Seagate/swift,psachin/swift,notmyname/swift,shibaniahegde/OpenStak_swift,openstack/swift,notmyname/swift,matthewoliver/swift,shibaniahegde/OpenStak_swift,zackmdavis/swift,psachin/swift,redbo/swift,Seagate/swift,smerritt/swift,prashanthpai/swift,swiftstack/swift,matthewoliver/swift,mjzmjz/swift,Akanoa/swift,scality/ScalitySproxydSwift,larsbutler/swift,openstack/swift,maginatics/swift,hurricanerix/swift,smerritt/swift,clayg/swift,AfonsoFGarcia/swift,levythu/swift,scality/ScalitySproxydSwift,thiagodasilva/swift,smerritt/swift,bradleypj823/swift,psachin/swift,hurricanerix/swift,prashanthpai/swift,rackerlabs/swift,zackmdavis/swift,openstack/swift,eatbyte/Swift,tipabu/swift,hbhdytf/mac2,AfonsoFGarcia/swift,hbhdytf/mac2,hurricanerix/swift,mjwtom/swift,daasbank/swift,mjwtom/swift,Akanoa/swift,bouncestorage/swift,hbhdytf/mac,wenhuizhang/swift,tipabu/swift,matthewoliver/swift,nadeemsyed/swift,eatbyte/Swift,rackerlabs/swift,IPVL/swift-kilo,dpgoetz/swift,sarvesh-ranjan/swift,hbhdytf/mac2,williamthegrey/swift,hbhdytf/mac,tipabu/swift,takeshineshiro/swift,swiftstack/swift,wenhuizhang/swift,bradleypj823/swift,bkolli/swift,revoer/keystone-8.0.0,matthewoliver/swift,Em-Pan/swift,aerwin3/swift,thiagodasilva/swift,swiftstack/swift,psachin/swift,larsbutler/swift,redhat-openstack/swift,openstack/swift,nadeemsyed/swift,IPVL/swift-kilo,redbo/swift,notmyname/swift