Remove dead code in PushManager and atomically update refs in EndPush.

org.geogit.web.api.commands.PushManager had a multimap keyked by remote IP
address whose values were supposedly all the object ids that have been
pushed, but they were never used (nobody calls PushManager.addObject() ), and
hance PushManager.alreadyPushed() is useless.

Removed all the dead code and made it so that PushManager.connectionSucceeded(...)
uses a geogit transaction to atomically update all the refs.