Should generate txout nonces deterministically

Makes testing easier and reduces dependency on strong random sources.

We only blind outputs when we have an unlocked wallet currently (I think) so we could just do a tagged hash of the commitment and the master wallet blinding key.

Unsure what we should use for the blinding factors themselves. A hash of the scriptPubKey + amount could work, but would result in equal commitments when people reused addresses, which we ought not to compromise privacy on even more..