Today we use the version of a DirectoryReader as a component of the key
of IndicesRequestCache. This usage is perfectly fine since the version
is advanced every time a new change is made into IndexWriter. In other
words, two DirectoryReader with the same version should have the same
content. However, this invariant is only guaranteed in the context of a
single IndexWriter because the version is reset to the committed version
value when IndexWriter is re-opened.

Since elastic#33473, each IndexShard may have more than one IndexWriter, and
using the version of a DirectoryReader as a part of the cache key can
cause IndicesRequestCache to return stale cached values. For example, in
elastic#27650, we rollback the engine (i.e., re-open IndexWriter), index new
documents, refresh, then make a count request, but the search layer
mistakenly returns the count of the DirectoryReader of the previous
IndexWriter because the current DirectoryReader has the same version to
the old DirectoryReader. This is possible because these two readers come
from different IndexWriters.

This commit replaces the the version with the reader cache key of
IndexReader as a component of the cache key of IndicesRequestCache.

Closes elastic#27650
Relates elastic#33473