Move Tx ready call from ShardWriteTransaction to Shard

Move Tx ready call from ShardWriteTransaction to Shard

On ready, the ShardWriteTransaction actor calls ready on the
ReadWriteShardDataTreeTransaction to obtain the ShardDataTreeCohort
and passes it with the ForwardedReadyTransaction message sent to
the Shard. Internally the ReadWriteShardDataTreeTransaction accesses
its parent, which is either the ShardDataTree or a ShardDataTreeTransactionChain,
to obtain the ShardDataTreeCohort. The parent holds unsynchronized
internal Shard state and is potentially unsafe to indirectly leak to the
transaction actor. This isn't an issue with the ShardDataTree b/c
finishTransaction doesn't access internal state but
ShardDataTreeTransactionChain does.

To be safe, we need to move the ready call to the Shard. Therefore, the
ReadWriteShardDataTreeTransaction instance is now passed with the
ForwardedReadyTransaction and the ShardCommitCoordinator now calls
ready.

Change-Id: I59295936944326b68506380519f8ade3b11164d9
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>