Check for change before container replicator updates db

As described in the related bug report, unnecessary updates
to the container db during replication can impact object
object GET performance in certain circumstances.

This patch changes swift/container/replicator.py so that
calls to merge_timestamps and update_reconciler_sync
are conditional on values having actually changed.

Related-Bug: 1332025
Change-Id: If498251656500ed7a3d7ca4b109ea1079b8513c2
Merge "Check for change before container replicator updates db"