Clustering: Handle Shard transaction backwards compatibility

Recent changes to the Shard 3-phase commit handling makes it incompatible with base Helium. We need to maintain backwards compatibility.

The incompatibility is a result of a transactionId field added to the CanCommitTransaction, CommitTransaction, AbortTransaction protobuf messages.

Solution:

• In CanCommitTransaction, CommitTransaction, AbortTransaction protobuf messages make the transactionId field optional.

• Add an optional version # field to the CreateTransaction protobuf message. A base Helium node wouldn't set the version so it will be 0.

• Pass the CreateTransaction version # to the ShardTransaction.

• On ReadyTransaction message, the ShardTransaction sends the version # with the ForwardedReadyTransaction message to the Shard.

• In the Shard, on ForwardedReadyTransaction, if the version # < 1 (Helium-1 version), create a BackardsCompatibleThreePhaseCohort actor with the transactionId and pass its actor ref back in the response. Otherwise pass the Shard actor ref back as it currently does.

• In BackardsCompatibleThreePhaseCommitCohort, on CanCommitTransaction, CommitTransaction, AbortTransaction messages, since the transactionId will be null or empty, create a new message with the cached transactionId and forward it to the Shard. Everything should work as is from there.