Summary:
We have a request to use the cold tier as primary source of truth for the DB, and to best support such use cases and to complement the existing options controlling SST file temperatures, we add two new DB options:
* `metadata_write_temperature` for DB "small" files that don't contain much user data
* `wal_write_temperature` for WALs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12957
Test Plan: Unit test included, though it's hard to be sure we've covered all the places
Reviewed By: jowlyzhang
Differential Revision: D61664815
Pulled By: pdillinger
fbshipit-source-id: 8e19c9dd8fd2db059bb15f74938d6bc12002e82b
aaa570b21f