Search: parse terms filters on a single term as a term filter.

Running a terms filter on a single term is equivalent to loading a postings
list into a bit set and then returning the bit set instead of reading the
postings list on the fly.

Close #9014
Core: Terms filter lookup caching should cache values, not filters.

The terms filter lookup mechanism today caches filters. Because of this, the
cache values depend on two things: the values that can be found in the lookup
index AND the mapping of the local index, since changing the mapping can change
the way that the filter is parsed. We should make the cache depend solely on
the content of the lookup index.

For instance the issue I was seeing was due to the following scenario:
 - create index1 with _id indexed
 - run terms filter with lookup, the parsed filter looks like `_id: 1 OR _id: 2`
 - remove index1
 - create index1 with _id not indexed
 - run terms filter without lookup, the parsed filter is `_uid: type#1 OR _uid: type#2` (the _id field mapper knows how to use the _uid field when _id is not indexed)
 - run terms filter with lookup, the filter is fetched from the cache: `_id: 1 OR _id: 2` but does not match anything since `_id` is not indexed.

Close #9027
Parent/child: Fix concurrency issues of the _parent field data.

`_parent` field data mistakenly shared some stateful data-structures across
threads.

Close #8396
Core: `ignore_unavailable` shouldn't ignore closed indices if a single index is specified in a search or broadcast request.

Closes #9047
Closes #7153
[TESTS] Make sure to wait for all shards to be allocated before running the test.