Internal cleanup

1. Moved DeployMessage's into its own DeployMessages object. 

2. Renamed MasterState to MasterStateResponse and WorkerState to WorkerStateResponse for clarity.

3. Removed Cache and SoftReferenceCache since they are no longer used.

4. Moved block manager messages into BlockManagerMessages object.

5. Moved all StandaloneClusterMessage's into StandaloneClusterMessages object.