[RaftLogEngine] Make log recycling effective when starting in cold by `prefill-for-recycle`.

[LykxSassinator]

Development Task

In previous work, we've already introduced enable-log-recycle for reusing the obsolete raft logs for newly append entries. But this feature is not effective when starting with an empty db, only making sense when the count of logs up to threshold.

So, we want to introduce a new trigger, named with prefill-for-recycle, to enable this feature in advance when starting TiKV with an empty db, which has been done in tikv/raft-engine#278.