feat(rocksdb): Bump rocksdb to v6.6.4

[acelyc111]

What problem does this PR solve?

Upgrade RocksDB to v6.6.4
#468

What is changed and how it works?

No changes.

Performance Comparation

1.12.1+prefix 版本	读写比	运行时长	读QPS	读Avg延迟	读P99延迟	写TPS	写Avg延迟	写P99延迟
(1)数据加载: 3客户端*10线程	0:1	1.99	-		-	41876	714	2985
(2)读写同时: 3客户端*15线程	1:3	1.28	16276	303	818	48828	816	4061
(3)读写同时: 3客户端*30线程	30:1	0.31	260147	322	664	8670	715	3227
(4)数据只读: 6客户端*100线程	1:0	0.11	794246	777	2819	-	-	-

升级rocksdb 版本	读写比	运行时长	读QPS	读Avg延迟	读P99延迟	写TPS	写Avg延迟	写P99延迟
(1)数据加载: 3客户端*10线程	0:1	1.98	-	-	-	41366	709	3565
(2)读写同时: 3客户端*15线程	1:3	1.29	16476	312	1041	48318	821	4355
(3)读写同时: 3客户端*30线程	30:1	0.29	273243	301	695	9042	660	3017
(4)数据只读: 6客户端*100线程	1:0	0.1	809753	690	2023	-	-	-

Check List

Tests

• Unit test
  Yes
• Integration test
  Yes
• Manual test (add detailed scripts or steps below)
  compatibility Test
  Steps:

1. Install Pegasus with old version V1 (with data version = 1, rocksdb version = v5.9.2)
2. Write data set A
3. Check data set A
4. Upgrade to current version (with data version = 1, rocksdb version = v6.6.4)
5. Write data set B
6. Check data set A and B
7. Downgrade to old version V1
8. Check data set A and B

Code changes

• Has exported function/method change
  No
• Has exported variable/fields change
  No
• Has interface methods change
  No
• Has persistent data change
  Yes

Side effects

• Possible performance regression
  No
• Increased code complexity
  No
• Breaking backward compatibility
  No

Related changes

• Need to cherry-pick to the release branch
  No
• Need to update the documentation
  Yes
• Need to be included in the release note
  Yes