[Remote Store] Integrate translog download in failover (replica-primary promotion)

[ashking94]

Is your feature request related to a problem? Please describe.
For remote backed indices with remote translog enabled, the incoming write request by the client gets served by the primary shard. In a pure local storage backed index, when a write request lands on the primary, the same is stored as a translog and then request fans out to all eligible replicas where the request is replayed similarly as the primary. With remote store backed translog, the translog is stored only on the primary along with remote store for durability and replicas are not subjected to translog. With this, now whenever a replica to primary promotion happens, there is a need for the translog to be downloaded from remote store and be played against the engine.

Describe the solution you'd like
Integrate remote translog download with failover.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[saratvemulapalli]

@ashking94 this is labelled 2.5, code freeze is 1/10 do you think you can make it?

[ashking94]

@saratvemulapalli this will be closed before the code freeze.