Bug 509: Added support for merge operation to InMemoryData Store

Bug 509: Added support for merge operation to InMemoryData Store

Merge operation support is critical for backwards compatibility
and for Binding-Aware mapping and Restconf, which are not aware
of Mixin nodes representing lists, leaflists and choices.

Semantics of merge operation are similar to subtree modified,
but when node is not existing it creates one based on
state provided by user of the Data Broker APIs.

Merge operation also fixes concurrent creations of mixin nodes
as demonstrated in ConcurrentImplicitCreateTest,
where two separate applications may indirectly create same
root node.

Change-Id: I9ab290b512601bd99750a68ee53564e67ef0c879
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>