aka "BOLT 3: Use revocation key hash rather than revocation key",
which builds on top of lightningnetwork/lightning-rfc#105 "BOLT 2,3,5:
Make htlc outputs of the commitment tx spendable with revocation key".
This affects callers, since they now need to hand us the revocation
pubkey, but commit_tx has that already anyway.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>