Move raw_password operations into a background process.

A minimal first step towards #291, this PR moves the Client operations of the raw_password API into a background compute cluster.  It's still far from production ready, but should avoid blocking of the event-loop due to e.g. pbkdf2 calculation.  If we can land something like this, then we should push the rest of #291 to a far-future milestone in the expectation that this API will go away in future.

@dannycoates r?