CDS: add backend support for local transactions

Add the message for submitting a DataTreeModification from the frontend
and the logic to apply it to the Shard's DataTree instance. This is
similar to BatchedModifications, except is is always considered ready.

The ReadyLocalTransaction is not directly serializable, but we create a
new serializer, which turns it into BatchedModifications when
serializing to byte stream.

Change-Id: I23a39c7b5997b48a355af73287d19bbf3ab0ae20
Signed-off-by: Robert Varga <rovarga@cisco.com>