Summary:
This is an effort to club all string related utility functions into one common place, in string_util, so that it is easier for everyone to know what string processing functions are available. Right now they seem to be spread out across multiple modules, like logging and options_helper.

Check the sub-commits for easier reviewing.
Closes https://github.com/facebook/rocksdb/pull/2094

Differential Revision: D4837730

Pulled By: sagar0

fbshipit-source-id: 344278a