change the init parameter for FileDescriptor

Summary:
  fix a bug in improve_file_key_search, change the parameter for FileDescriptor

Test Plan:
  make all check

Reviewers: sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D19611
JSON (Document) API sketch

Summary:
This is a rough sketch of our new document API. Would like to get some thoughts and comments about the high-level architecture and API.

I didn't optimize for performance at all. Leaving some low-hanging fruit so that we can be happy when we fix them! :)

Currently, bunch of features are not supported at all. Indexes can be only specified when creating database. There is no query planner whatsoever. This will all be added in due time.

Test Plan: Added a simple unit test

Reviewers: haobo, yhchiang, dhruba, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18747
Add missing DB functions to DocumentDB