RFC:TiKV RawKV MVCC GC

[ystaticy]

Summary

Move TiKV MVCC GC worker from TiDB into a group of independent GC worker node role and implement a new GC process in TiKV for RawKV.

Motivation

GC worker is an important component for TiKV that deletes outdated MVCC data so as to not explode the storage. But currently, the GC worker is implemented in TiDB, which makes TiKV not usable without TiDB.And current GC process is just for transaction of TiDB,it's not usable for RawKV.

[BusyJay]

The RFC is quite simple yet. For example, how is the new node designed? How it triggers GC? How does GC work in details? What's the triggering frequency?

[ystaticy]

@BusyJay OK,I'll add more details for this RFC