Optimisation: reduce the number of RPCs when using pessimistic locking

Right now, with pessimistic locking:

TransactionManager.begin()
Object k = cache.put(k,v); //this performs two RPCs to same nodes: a ClusterGetCommand and an LockControlCommand.

The two RPCs can be merged together.
Whilst this is an optimisation, it is quite an important one as it has an major impact on performance on every put.