Summary:
Rename `_header` to `header_` to align with the C++ naming scheme we use.
Rename `readKey` to `readUnsignedShort` as purpose of the method have changed.
Changelog: [Internal]
Reviewed By: javache
Differential Revision: D33637127
fbshipit-source-id: a82f4d6c1b753b21e0567fbe919af98e4c78105d