XContentHelper.mergeDefaults with single key object list ([ { key: .. } , {key : ... } .. ] ) didn't add defaults which were not already in content

 Closes #3538
Improved multi percolate api, to bundle all the requests by shard into one shard level request instead of sending each percolate request separately to the shards.
Add the /_cat/count and /_cat/count/{index} API.

The cat count API allows retrieving the count of all docs in the
cluster, or docs in specific indices, in a human-readable format.