Support 'flashback' request

[bb7133]

Ref:

• Supports FLASHBACK TO TIMESTAMP in TiDB pingcap/tidb#37197
• Tracking Issue for Flashback tikv#13303

We plan to support an MVCC-based flashback feature for TiDB, as described in the issues above. The proposed RPC request:

message FlashbackToVersionRequest {
    Context context = 1;
    // The TS version which the data should flashback to.
    uint64 version = 2;
    bytes start_key = 3;
    bytes end_key = 4;
}

message FlashbackToVersionResponse {
    errorpb.Error region_error = 1;
    string error = 2;
}

We would like to implement the new interface based on it:

func flashback(keyRanges []kv.KeyRange, version int64) (status Status, err error)

In which:

• keyRanges: a series of non-overlaped start/end key pairs that need to be flash-backed by TiKV.
• version: the timestamp of MVCC version that need to be flash-backed by TiKV.
• status: the status of flashback request: if TiKV is doing the flashback job, it returns the total number of regions and the number of done regions; if the job is not started by TiKV, it starts running the job.