Summary:
In mongo, we currently have a single column family and I'd like to support setting rocksdb::Options from string. This diff provides an option to GetOptionsFromString()

There's one more problem. Currently GetColumnFamilyOptionsFromString() overwrites block_based options. In mongo I set default values for block_cache and some other values of BlockBasedTableOptions and I don't want them reset to default with GetOptionsFromString().

Test Plan: added unit test

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33729