Search & Count: Add an option to early terminate document collection

The idea is to add an option which will let the user control the number of matched documents collected per shard before scoring (if applicable). This will be helpful for exists-type functionality or when not all matched document has to be scored.

Closes #6876 
I actually wonder if we should have a `MaybeBoolean` class that implements `ToXContent` and can do these kind of merge operations ie similar of haskel maybe