Return local shard's DataTree from ActorContext#findPrimaryShardAsync

Return local shard's DataTree from ActorContext#findPrimaryShardAsync

For upcoming patches to optimize local shard transactions, we need
access to the Shard's local DataTree in the TransactionProxy. This can
be returned by ActorContext#findPrimaryShardAsync along with the shard's
ActorSelection.

I added a new class PrimaryShardInfo that encapsulates the shard's
ActorSelection and an Optional whose value contains the Shard's DataTree
if the shard actor is local to the caller.

ActorContext#findPrimaryShardAsync now returns a Future<PrimaryShardInfo>.
Currently, the PrimaryShardInfo contains an absent Optional but will be
implemented to contain the local Shard's DataTree in a subsequent patch.
Another subsequent patch will utilize the DataTree in the
TransactionProxy.

Change-Id: I9bd6425d597c307391d09554fe9ccebd03593180
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>