RocksDBOptionsParser::Parse()'s `ignore_unknown_options` argument only ingores options from higher version.

Summary:
RocksDB should always be able to parse an option file generated using the same or lower version. Unknown option should only happen if it is from a higher version. Change the behavior of RocksDBOptionsParser::Parse()'s behavior with ignore_unknown_options=true so that unknown option from a lower or the same version will never be skipped.
Closes https://github.com/facebook/rocksdb/pull/3527

Differential Revision: D7048851

Pulled By: siying

fbshipit-source-id: e261caea12f6515611a4a29f39acf2b619df2361