Made sure the new cluster state is not held by AckCountDownListener

(also made AckCountDownListener class static)
Added aggs option to percolate api documentation.
Use a dedicated cluster name for GeoDistanceSearchBenchmark (like we do for other benchmarks).
Added more logging for percolator.
Lowered log level in percolator tests.
Preference only_node with unknown nodeId returns useful exception

When the search preference is set to only node, but this node is not a
data (or does not exist), we return a search exception, which indicates,
that this is actually a server problem.

However specifying a non-existing node id is a client problem
and should return a more useful error message than
{"error":"SearchPhaseExecutionException[Failed to execute phase [query_fetch], all shards failed]","status":503}