Delete conflicting logs

• When appending logs to a follower/learner, conflicting logs has be removed.
• When installing snapshot(another form of appending logs), conflicting logs should be removed.

Why

1. Keep it clean

The first reason is to keep logs clean: to keep log ids all in ascending order.

2. Committed has to be chosen

The second reason is to let the next leader always choose committed logs.

If a leader commits logs that already are replicated to a quorum, the next leader has to have these log. The conflicting logs on a follower A may have smaller log id than the last log id on the leader. Thus, the next leader may choose another node B that has higher log than node A but has smaller log than the previous leader.

3. Snapshot replication does not have to delete conflicting logs

See: Deleting-conflicting-logs-when-installing-snapshot

Deleting conflicting logs when installing snapshot is only for clarity.