make rocksdb format version configurable

[hangc0276]

Motivation

Fix #2823
RocksDB support several format versions which uses different data structure to implement key-values indexes and have huge different performance. https://rocksdb.org/blog/2019/03/08/format-version-4.html

https://github.com/facebook/rocksdb/blob/d52b520d5168de6be5f1494b2035b61ff0958c11/include/rocksdb/table.h#L368-L394

  // We currently have five versions:
  // 0 -- This version is currently written out by all RocksDB's versions by
  // default.  Can be read by really old RocksDB's. Doesn't support changing
  // checksum (default is CRC32).
  // 1 -- Can be read by RocksDB's versions since 3.0. Supports non-default
  // checksum, like xxHash. It is written by RocksDB when
  // BlockBasedTableOptions::checksum is something other than kCRC32c. (version
  // 0 is silently upconverted)
  // 2 -- Can be read by RocksDB's versions since 3.10. Changes the way we
  // encode compressed blocks with LZ4, BZip2 and Zlib compression. If you
  // don't plan to run RocksDB before version 3.10, you should probably use
  // this.
  // 3 -- Can be read by RocksDB's versions since 5.15. Changes the way we
  // encode the keys in index blocks. If you don't plan to run RocksDB before
  // version 5.15, you should probably use this.
  // This option only affects newly written tables. When reading existing
  // tables, the information about version is read from the footer.
  // 4 -- Can be read by RocksDB's versions since 5.16. Changes the way we
  // encode the values in index blocks. If you don't plan to run RocksDB before
  // version 5.16 and you are using index_block_restart_interval > 1, you should
  // probably use this as it would reduce the index size.
  // This option only affects newly written tables. When reading existing
  // tables, the information about version is read from the footer.
  // 5 -- Can be read by RocksDB's versions since 6.6.0. Full and partitioned
  // filters use a generally faster and more accurate Bloom filter
  // implementation, with a different schema.
  uint32_t format_version = 5;

Different format version requires different rocksDB version and it couldn't roll back once upgrade to new format version

In our current RocksDB storage code, we hard code the format_version to 2, which is hard to to upgrade format_version to achieve new RocksDB's high performance.

Changes

1. Make the format_version configurable.

[eolivelli]

Lgtm

[merlimat]

LGTM. Since RocksDB takes a config file, we should also let users configure that, so that we don't need new code changes to support new options.

[Myasuka]

Shall we add some comments or description here? I think not every customer could have knowledge that this format version could have impacts on performance.