Use HeaderKind instead of magic in RefData

Summary: Parseable headers allow self-identifying all heap objects, which means
we can get rid of RefData::Magic but retain the asserts.

Reviewed By: @bertmaher

Differential Revision: D1753084

Signature: t1:1753084:1419105121:3c838d8ab2a9ba6ed5dde06e806db9c349501eba