Clarify the wait_index policy in replication RPC

Summary

• fix: must not call wait_index in replication related RPC

Remove the corresponding calls, add module-level doc-comments with explanations on what the hell is going on.

This also fixes some flaky tests including #1294 (closed)

• Revert "feat: add wait_index to proc_replication_demote"

Turns out blocking inside proc_replication_demote is not safe, as it may lead to deadlocking if a replicaset master switchover happens during a replicaset catch-up via raft snapshot. The symptom of the problem was the flakiness of the tests (for example issue #1294 (closed)).

Closes #1294 (closed)

This reverts commit ec2b760a.

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• Cherry-pick to: none
• Docs follow-up: not necessary
• Changelog is updated