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