TiFlash can't gc totally after set tiflash replica 0

[lilinghai]

Bug Report

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v6.0

[lidezhu]

Reproduce it locally. The remaining data doesn't correspond to any region range in the tiflash node. And this is a possible case in the cluster, and tikv handle this case by remove garbage ranges at restart: https://github.com/tikv/tikv/blob/b5559844ce26a453900ca64c86902647dc807e28/components/raftstore/src/store/fsm/store.rs.

So this is a normal behavior and is not a bug.

The remaining data is in the range: [216172782114912251, 216172782117644718]
The two neighbor region ranges ever on this node are: [144115188085470701, 216172782114912251) and [216172782117644721, 216172782121487120).

tiflash log:
https://drive.google.com/file/d/1PY5F-STVXPDSqj2alhvNC-PaMyH7Zl-e/view?usp=sharing
https://drive.google.com/file/d/11vVeoR_rlpoduxAKFvVZkhtHeGRqkh3O/view?usp=sharing
https://drive.google.com/file/d/1JP9-Mi0YcfGtLeye9sRCg2232DltDWza/view?usp=sharing

[lidezhu]

This pr #4500 just fix the following two problems:

1. some empty segments cannot be merged;
2. the stable size is not 0 for empty dmfile because there is a '0' in every index file in such case;

The problem that some data left on tiflash node when all region is removed will be fixed later. @CalvinNeo

[CalvinNeo]

Sending snapshots may change start_key and end_key, TiKV will delete range regularly, or at least before shutdown. However, these messages will not be delivered to TiFlash.