Use as less engine_rocks in engine_tiflash as we can

[CalvinNeo]

Signed-off-by: CalvinNeo calvinneo1995@gmail.com

What problem does this PR solve?

Issue Number: ref pingcap/tiflash#5170

Problem Summary:
We must own a engine_rocks::RocksEngine, since TiKV has not decouple from engine_rocks yet.

However, we will try to use as less engine_rocks in engine_tiflash as we can, since Proxy will not pick every new features while TiKV is upgrading its engine_rocks.

The structs that we must import from engine_rocks are:

1. RocksDBVector
2. RocksEngineIterator
3. RocksSnapshot

If we need to keep track of engine_rocks, we can just merge codes from engine_rocks to engine_tiflash, this won't happen frequently, since we don't need much of new features from engine_rocks. We adopt them only for compation.

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