Summary:
The current way of implementing CompressionOptions.parallel_threads introduces a format change. We plan to change CompressionOptions's serailization format to a new JSON-like format, which would be another format change. We would like to consolidate the two format changes into one, rather than making some users to change twice. Hold CompressionOptions.parallel_threads from being supported by option string for now. Will add it back after the general CompressionOptions's format change.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6782

Test Plan: Run all existing tests.

Reviewed By: zhichao-cao

Differential Revision: D21338614

fbshipit-source-id: bca2dac3cb37d4e6e64b52cbbe8ea749cd848685