Meta region leader transfer may cause latency spike

[overvenus]

[2022/03/01 19:09:54.620 +08:00] [INFO] [raft.rs:2300] ["[term 7] received MsgTimeoutNow from 32833 and starts an election to get leadership."] [from=32833] [term=7] [raft_id=32835] [region_id=32832]
[2022/03/01 19:09:54.620 +08:00] [INFO] [raft.rs:1513] ["starting a new election"] [term=7] [raft_id=32835] [region_id=32832]
[2022/03/01 19:09:54.620 +08:00] [INFO] [raft.rs:1112] ["became candidate at term 8"] [term=8] [raft_id=32835] [region_id=32832]
[2022/03/01 19:09:54.620 +08:00] [INFO] [raft.rs:1267] ["broadcasting vote request"] [to="[32833, 32834]"] [log_index=24404] [log_term=7] [term=8] [type=MsgRequestVote] [raft_id=32835] [region_id=32832]
[2022/03/01 19:09:54.629 +08:00] [INFO] [raft.rs:2171] ["received votes response"] [term=8] [type=MsgRequestVoteResponse] [approvals=2] [rejections=0] [from=32834] [vote=true] [raft_id=32835] [region_id=32832]
[2022/03/01 19:09:54.629 +08:00] [INFO] [raft.rs:1196] ["became leader at term 8"] [term=8] [raft_id=32835] [region_id=32832]
[2022/03/01 19:09:54.629 +08:00] [INFO] [peer.rs:3730] ["require updating max ts"] [initial_status=34359738580] [region_id=32832]
[2022/03/01 19:09:54.629 +08:00] [INFO] [deadlock.rs:735] ["became the leader of deadlock detector!"] [self_id=4]
[2022/03/01 19:09:54.629 +08:00] [INFO] [pd.rs:1060] ["succeed to update max timestamp"] [region_id=32832]
[2022/03/01 19:09:54.636 +08:00] [INFO] [endpoint.rs:309] ["register observe region"] [region="id: 32832 end_key: 7480000000000000FF175F728000000000FF000DE10000000000FA region_epoch { conf_ver: 5 version: 106 } peers { id: 32833 store_id: 1 } peers { id: 32834 store_id: 5
[2022/03/01 19:09:54.640 +08:00] [INFO] [endpoint.rs:233] ["Resolver initialized"] [pending_data_index=0] [snapshot_index=24405] [observe_id=ObserveID(21611)] [region=32832]

# Lots of following logs.
[2022/03/01 19:09:54.941 +08:00] [INFO] [service.rs:96] ["cdc connection version"] [downstream="\"ipv4:172.16.4.34:46088\""] [features=BATCH_RESOLVED_TS] [version=5.1.4]
[2022/03/01 19:09:54.941 +08:00] [INFO] [endpoint.rs:538] ["cdc register region"] [downstream_id=DownstreamID(29680)] [observe_id=ObserveID(21614)] [req_id=18234] [conn_id=ConnID(20939)] [region_id=32832]

Similar to #4756