Bug 509: Improve conflict detection of leaf merges

Bug 509: Improve conflict detection of leaf merges

Original conflict detection for merge action on leaves
only looked at node version and subtree version, which
caused the transaction to fail even if the leaf was
unmodified and contained the same value.

Conflict detection now checks if the leaves are not same,
and only in that case it checks for concurrent writes.

Change-Id: I5e43233e86adf96e3dad2a93cdcf512b3d17977a
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>