fix percolate stats tests failures
Zen Discovery Cluster Events to have Priority.URGENT
Master node cluster state events resulting in zen discovery (node gets added, removed, for example) should be processed with priority URGENT as its always better to process them as fast as possible, and not let other events get in the way.
closes #3361
reroute post shard started should have HIGH prio as well
Test to some extent the time spent on percolating.
add ability to associate a timeout with a priority executor
enhancement that can be used later to timeout tasks that are pending, also added the ability to get the pending task list from the executor
Transport: Add a dedicated ping channel
Today, we have a low/med/high channel groups in our transport layer. High is used to publish cluster state and send ping requests. Sometimes, the overhead of publishing large cluster states can interfere with ping requests.

Introduce a new, dedicated ping channel (with size 1) to have a channel that only handles ping requests.
closes #3362
catch failures and notify the listener in aliases action handling
Fix ConcurrentPercolatorTests replaces CountDownLatches array by a Semaphore.

The writes to the CountDownLatches array wasn't visible by all threads when the countdown latch array slots were re-initialized.
Also serialize the routing and preference
Added alias action validation

Moved alias action validation to IndicesAliasesRequest, so that Java api and RestIndexPutAliasAction can benefit from it too.
Added check in MetaDataIndexAliasesService too.

Closes #3363
Cluster State Update APIs (master node) to respect master_timeout better
Currently, the master node might be processing too many cluster state events, and then be blocked on waiting for its respective even to be processed. We can use the new cluster state update timeout support to use the master_timeout value and respect it.

closes #3365
Cluster State Update APIs (master node) to respect master_timeout better
also respect the timeout when trying to obtain the md lock
relates #3365
should be 500...
Add FastVectorHighlighter support for more complex queries.

FastVectorHighlighter fails at highlighting some complex queries such as
multi phrase queries which have two terms at the same position. This can be
easily triggered by running a `match_phrase` query with an analyzer which
outputs synonyms such as SynonymFilter or WordDelimiterFilter.

Close #3357
move master actions to accept a lister, improve cluster service state execution
- master actions many times end up being executed on the cluster service, so there is no need to block them on the management thread pool to wait for a response, this remove the load on the management thread pool, and also simplifies the code implementing it
- cluster service state update exception handling was improved to include a callback when a failure happens during state update execution, this makes sure that we catch all relevant exceptions and invoke the callback, as well as simplify the code of cluster state update tasks, as they can throw failures from within the execute method and then handle them properly
Fix SimplePercolatorTests#testPercolateStatistics
Wait a bit longer for ttl to have occurred
make sure we update last failure
Add optimize thread pool (size 1) dedicated to perform explicit optimize API
Have a dedicated thread pool for explicit optimize calls (shard level optimize operations). By default, the size should be 1 to work the same with how things work currently allowing for only 1 shard level optimize on a node.

The change allows to see the optimize thread pool stats now, and potentially increase the thread pool size for beefy machines.

closes #3366
Upgrade to Lucene 4.4