Bug 3194: Dynamically update PrimaryShardInfo cache when leader changes

Bug 3194: Dynamically update PrimaryShardInfo cache when leader changes

Added a PrimaryShardInfoFutureCache class which maintains the cache of
PrimaryShardInfo Future instances per shard. This code was moved from the ActorContext.

The PrimaryShardInfoFutureCache instance is created by the
DistributedDataStore and passed to the ShardManager, as well as to the
ActorContext. The ActorContext uses the cache as before.

On ShardLeaderChanged message, if the leaderId has changed, the
ShardManager calls remove on the PrimaryShardInfoFutureCache to
invalidate the entry for the shard name.

As part of this patch some refactoring was done which removed ShardInfoListeners.

There are a few miscellaneous changes in this patch as well,
- Create the appropriate type of transaction on the datatree for local transactions
- Tracking only read-only transactions which phantom references

Change-Id: I4c7a3845e311e5130d80f22f7cfb3c82c5ad731d
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>