Major Description
When profiling graph computations running on HAMA it turned out that huge amount of time is spent in RPC calls.
To reduce this overhead I propose extending the GroomServer's API with a put(BSPMessageBundle) method so that a group of messages can be transferred from the caller to the callee at once. Also, the sync() operation should be improved by sending messages in batches rather than one-by-one.
Using BSPMessageBundle makes it easier to add support for compression of messages later.
