Use the same rocksdb snapshot when sending snapshot

[critical27]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Description:

This is some prepare work for later data balance test, mainly two changes:

1. Follow up of Add snapshot for prefix iterate. #3717(In that pr we use snapshot when doing data scan, but the commit log id/term is not taken care). So in this PR, we create a rocksdb snapshot at first, read commit log id/term, and use the same snapshot to scan data.
2. We can't simply skip the special raft command, because if we skip the special log after rebooting, the state of raft peer will be wrong.
3. Add some metrics in raft, remove the SlowOpTracker.

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

not related

[wenhaocs]

LGTM. Want to make sure deleting SlowOpTracker is also intended.

[critical27]

LGTM. Want to make sure deleting SlowOpTracker is also intended.

Yeah, SlowOpTracker is only used in raft, and after log level is adjusted, SlowOpTracker still use LOG(INFO) which is not appropriate. Since we could add more metrics, just unify them and delete the SlowOpTracker.

[panda-sheep]

Good job！