BUG 2138: Introduce DistributedShardedDOMDataTree

This is the initial patch that introduces the concept of
prefix based shards and consumer/producer api's into the
DistributedDatastore.

Each shard has a frontend registered into the ShardedDOMDataTree
that forwards calls into the DistributedDataStoreClient frontend
which then handles the actual writes into the Shard.
These ShardFrontends are then distributed into other nodes
via ShardedDataTreeActor which also handles notification of
other nodes of open Producers.

Change-Id: Ifcbd1021fdaeac7929fc547e6e32be50da9d93fc
Signed-off-by: Tomas Cere <tcere@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>