fix test server/node read leader with unapplied log
fix test server/node read leader with unapplied log

fix test_read_leader_with_unapplied_log (prevent node 2 from step down to follower)

BusyJay/tikv,pingcap/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv