Summary:
Add methods and constructors for handling enums to the OptionTypeInfo.  This change allows enums to be converted/compared without adding a special "type" to the OptionType.

This change addresses a couple of issues:
- It allows new enumerated types to be added to the options without editing the OptionType base class (and related methods)
- It standardizes the procedure for adding enumerated types to the options, reducing potential mistakes
- It moves the enum maps to the location where they are used, allowing them to be static file members rather than global values
- It reduces the number of types and cases that need to be handled in the various OptionType methods
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6423

Reviewed By: siying

Differential Revision: D21408713

Pulled By: zhichao-cao

fbshipit-source-id: fc492af285d011822578b95d186a0fce25d35626