Don't use scrypt for sealing/unsealing secrets in the observer and auto-mount daemon, since the key we're using is sufficiently long that we don't need to rely on (or worry about) adversaries guessing it (short of guessing every 256-bit key).
This greatly speeds up credential generation and propagation.
To do so, we added crypto routines for symmetric encryption/decryption in such a way that we preserve the IV along with the ciphertext (renaming the original symmetric crypto methods that take an IV as an argument to differentiate them), and
updated the observer (and all affected clients) to use the new crypto routines in place of using the scrypt-derived password-based symmetric encryption scheme.  Part of this refactoring removes the the _UNSAFE tag, in favor of explicitly labeling
methods as _mlocked to be less ambiguous about the "unsafeness."

Unrelated, this commit also separates the observer secret from the observer config file--the secret gets put into a file of its own (as a 256-bit hex-encoded key) which will be loaded at runtime by the observer.  The config file simply references its path.