This starts to send commands that are found in the queue and allows commands to be synchronously sent via {@link #send(Command)}. Commands are placed in the queue by {@link #sendAsynch(Command,CommandResponseCallback)}. This method also puts commands on the queue that it finds in the persistence command store (for those commands that failed but need to be delivered).
If this client was not already {@link #isSending() sending}, this method returns true to indicate the client's mode has changed from stopped to started.
If this client is already {@link #isSending() sending}, this method does nothing and returns false to indicate the mode hasn't changed - it is still in the sending mode.
@return
true if this client was not sending prior to this method being called (and hence this clientchanged modes);
false if this client was already sending and thus this method ended up being a no-op