Simplify indices stats API

Note: This breaks backward compatibility

* Removed clear/all parameters, now all stats are returned by default
* Made the metrics part of the URL
* Removed a lot of handlers
* Added shards/indices/cluster level paremeter to change response serialization
* Returning translog statistics in IndicesStats
* Added TranslogStats class
* Added IndexShard.translogStats() method to get the stats from concrete implementation
* Updated documentation

Closes #4054