Performance optimizations for simple transactions

Performance optimizations for simple transactions

- Cache the ActorSelection for the primary shard in ActorContext
- Use isInstance to check for type of message instead of equals in ActorContext
  ShardManager and Shard
- Change the order in which we check for transaction type to create in Shard
- Create ShardTransactionIdentifier using the ShardTransactionIdentifier
  constructor instead of the Builder

These optimizations are aimed at making simple transactions faster in the dsBenchMark. This
set of optimizations reduced the amount of time that test takes from 20s to 18s on my Mac.

Change-Id: I1a4328d90839ef80041ae4f3bb7dccd45ac7fb97
Signed-off-by: Moiz Raja <moraja@cisco.com>