Prevented empty filters from causing NPE

 Closes #3724
Discovery to support a timeout waiting for other nodes to processing new cluster state
The master node processing changes to cluster state, and part of the processing is publishing the cluster state to other nodes. It does not wait for the cluster state to be processed on the other nodes before it moves on to the next cluster state processing job.

This is fine, we support out of order cluster state events using versioning, and nodes can handle those cases. It does lead though to non optimal API semantics. For example, when issuing cluster health, and waiting for green state, the master node will report back once the cluster is green based on its cluster state, but that mentioned "green" state might not have been received by all other nodes yet.

Add a discovery.zen.publish_timeout setting, and default it to 5s. This will give a best effort into making sure all nodes will process a cluster state within a window of time.

closes #3736
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.
Add asserting searcher to track searcher references

This assertion module also injects an AssertingIndexSearcher that
checks if our queries are all compliant with the lucene specification
which is improtant for future updates and changes in the upstream project.