Summary:
This is essentially a re-submission of #4251 with a few improvements:

- Split `CompressionDict` into two separate classes: `CompressionDict` and `UncompressionDict`
- Eliminated `Init` functions. Instead do all initialization work in constructors.
- Added test case for parallel DB open, which is the scenario where #4251 failed under TSAN.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4849

Differential Revision: D13606039

Pulled By: ajkr

fbshipit-source-id: 08c236059798c710db9cbf545fce0f371232d447