Add BlobHub.AddReceiveHook; clean up docs, clarify how to use BlobReceiver.

Change-Id: Icc5b55fcbabc852c7cacc9a806f12098d5e59377
encrypt: use key metaIndex for index, not meta; that was used. fixes devcam server.

Change-Id: I483c696dcc3402537bec136f8847c17767760b3b