JSONDocument

Summary:
After evaluating options for JSON storage, I decided to implement our own. The reason is that we'll be able to optimize it better and we get to reduce unnecessary dependencies (which is what we'd get with folly).

I also plan to write a serializer/deserializer for JSONDocument with our own binary format similar to BSON. That way we'll store binary JSON format in RocksDB instead of the plain-text JSON. This means less storage and faster deserialization.

There are still some inefficiencies left here. I plan to optimize them after we develop a functioning DocumentDB. That way we can move and iterate faster.

Test Plan: added a unit test

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18831