Move HTLCFailChannelUpdate handling out-of-band

Move HTLCFailChannelUpdate handling out-of-band

While this isn't neccessary for message ordering consistency, this
does mean that we won't end up processing an HTLCFailChannelUpdate
from a update_fail_htlc prior to it being fully committed (where if
the peer disconnects/reconnects it could theoretically give us a
different result, eg if their next-hop reconnected to them).