Summary:
This branch extends existing property map which keeps values in doubles to keep values in strings so that it can be used to provide wider range of properties. The immediate need for that is to provide IO stall stats in an easy parseable way to MyRocks which is also part of this branch.
Closes https://github.com/facebook/rocksdb/pull/2794

Differential Revision: D5717676

Pulled By: Tema

fbshipit-source-id: e34ba5b79ba774697f7b97ce1138d8fd55471b8a