Only expose safely stored data to reads

[ywelsch]

Dirty reads happen when an operation is returning results based on data that has not yet made it fully into the system (for example has only been indexed on the primary, but hasn't made it yet to the replicas, which means that if the primary fails, the data might be lost and the previously returned result would be incorrect). Eliminating dirty reads means that the user will never see data that has not been safely stored on all in-sync shard copies, i.e. the user will only see data that has made it fully into the system, and that the system will never henceforth silently lose nor revert.

Dirty reads affect the get, search, as well as indexing (CAS & update) APIs, and are documented in the Elasticsearch reference docs.

This issue is a place holder for addressing dirty reads on the three above-mentioned APIs.

Relates #20031

[elasticmachine]

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