Implement early termination for Count API, if threshold is provided

A new request param `threshold` is added, which will allow Count query to early terminate, once the number of matched docs is equal to `threshold` for every shard. This will be helpful for exists-type functionality.
maybe add more docs like `between(2, max)` and use `indexRandom()` so you get holes in the segments ie. del docs. Then you can can randomly set the threshold `between(2, max-1)` etc.