Introduce proxy_server and new-mock-engine-store

[CalvinNeo]

What problem does this PR solve?

Issue Number: ref pingcap/tiflash#5170

Problem Summary:

1. Introduce crate proxy_server for a decouple of component/server, it will be used to start a proxy service.
2. Introduce crate new-mock-engine-store as a new mock-engine-store plus a test_raftstore implementation for proxy. This will enable us to run mix-mode tests. Tests in tests/failpoints and tests/integrations are obsolete tests and will be removed once the refactor ends. New tests should be added in tests/proxy with support of new-mock-engine-store.
3. Introduce crate engine_tiflash as a new KvEngine. It currently just works like former RocksEngine, unless we enable compat_new_proxy. Since some PRs have not been merged in TiKV, we can only enable this feature in some tests in tests/proxy. It can not be enabled in formal codes, since it can filter some writes.
4. cherry pick newly merge TiKV PRs, aka on_empty_cmd.
5. Fix some bugs.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None