Added CompatibleOptions for compatibility with LevelDB Options

Summary: Created a CompatibleOptions object that can be used as a LevelDB Options object and then converted to a RocksDB Options object using the ConvertOptions() method.

Test Plan: Unit test included in diff.

Reviewers: ljin

Reviewed By: ljin

Subscribers: sdong, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28893