Added support for shard_size in terms agg

Closes #4242
Cleanup of IndexFieldDataService.getForField.

This method has 2 signatures and one of them is dangerous since it allows to
discard fielddata configuration of the field mapper. This commit changes the
percolator so that it uses fielddata configuration of the _id field mapper
instead of forcing the paged_bytes format.

Closes #4270
Fix assertions in testEquivalence and beef up the test
Catch AlreadClosedException in RobinSearcher#release()

When we relocate a shard we might still have pending SearchContext
instances hanging around that will be used in "in-flight" searches
on the already relocated shard. This is a valid operation but if
we have already closed the underlying directory which happens during
cleanup concurrently the close call on the IndexReader can trigger
an AlreadyClosedException when the NRT reader tries to cleanup files
via the IndexWriter.

Closes #4273
[DOCS] Documented rest.action.multi.allow_explicit_index
[DOCS] Fixed a bad link