ISPN-2787 NPE after ReplaceCommand

* Add MergeDuringReplaceTest to reproduce the issue.
* Fix one of the ControlledRpcManager.invokeRemotely methods that was ignoring the actual result and was always returning null.
* Minor logging improvements to be able to debug this issue.
* Remove harmfull optimization from RpcManagerImpl.invokeRemotely(Collection<Address> recipients, ReplicableCommand rpcCommand, ResponseMode mode, long timeout, boolean usePriorityQueue, ResponseFilter responseFilter) that was not going remotely and neither threw any exception if the destinations were no longer in cluster and current node was the only member.