Make sure the transport response handler is invoked only once.

There was a small window of time where the transport response handler's handException method was invoked twice. As far as I can tell this happened when node disconnect event was processed just after the request was registered and between a "Node not connected" error was thrown. The TransportService#sendRequest method would invoke the transport response handler's handException method regardless if it was already invoked. This resulted that for one request failure, two retries were executed.

The mpercolate api has an assert that tripped when more than the expected shard level responses were returned. This was caused by the issue described above. For the a single shard level request we had multiple responses and this broke the the the total excepted responses. Also the reduce could be started prematurely, which resulted in an incorrect final response (e.g. total count being incorrect). For example: two shards in total, shard 0 gets reduces twice. The second shard 0 response gets in just before shard 1 response gets in. The reduce starts without shard 1 response.