Summary:
Added code for generically handing structs to OptionTypeInfo.  A struct is a collection of variables handled by their own map of OptionTypeInfos.  Examples of structs include Compaction and Cache options.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6425

Reviewed By: siying

Differential Revision: D21668789

Pulled By: zhichao-cao

fbshipit-source-id: 064b110de39dadf82361ed4663f7ac1a535b0b07