Implement PreVote optimisation for Raft

[cjen1-msft]

Due to the liveness issue where a retiring replica never learns of its retirement, but still tries to join the service (#7334), it is a good time to start looking at implementing pre-vote.

This issue should be used to track this.

• Implement in TLA
• Implement in C++
• Trace Validation
• Migration plans
  • The constraint is that old nodes will at best ignore prevote messages. So mixed deployments will be unable to elect pre-vote candidates until a majority is migrated.

[achamayou]

Linking #3985, #2577 and #99, which are all resolved once this is complete.