[TEST] Added REST tests for _cat/thread_pool endpoint
[TEST] fixed typo in _cat/thread_pool docs
more AbstractRestResponseActionListener in RestThreadPoolAction (cat api)
[TEST] Made cat.count REST test more readable
Fix AndDocIdSet#IteratorBasedIterator to not violate initial doc state

AndDocIdSet#IteratorBasedIterator was potentially initialized with
NO_MORE_DOCS which violates the initial state of DocIdSetIterator and
could lead to undefined behavior when used in a search context.

Closes #5049