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