[TEST] Fixed get_source/60_realtime_refresh REST test to work against multiple nodes

Replaced also wait for yellow with wait for green when using no replicas.

get_source/60_realtime_refresh tests per shard refresh using refresh:true and realtime:true in get api. We might run into troubles though if we have a replica that gets initialized after a doc was indexed without a refresh, as that doc will be found when searching against that specific replica shard (as a refresh happens automatically before a replica gets exposed as started).
[TEST] moved routing REST tests to 0 replicas

REST tests get run against either 1 node or multiple nodes. Wait for yellow with replicas>0 is not enough when running against multiple nodes as replicas shard might get initialized  during testing, which can cause timing issues.
[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