Replace RefCountedFuture with something more robust

[DaveCTurner]

In #102498 we introduced RefCountedFuture as an expedient fix to the ref-counting of responses which are temporarily solely retained by a PlainActionFuture, as used by the blocking calls to AbstractClient. This is an ugly hack and likely to cause problems in future (e.g. if a timed .get() times out then we leak the ref). This issue tracks the work to remove this hack and replace it with something more robust, such as the proposal in #102507.

(or we could just stop using these blocking calls altogether, especially in production code where this sort of blocking is just not appropriate)

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)