daemon: handle HTLC as per BOLT #2 algorithm.

From BOLT#2 (rev 8ee09e749990a11fa53bea03d5961cfde4be4616):

   Thus each node (conceptually) tracks:
...
   3. Two *unacked changesets*: one for the local commitment (their proposals) and one for the remote (our proposals)
   4. Two *acked changesets*: one for the local commitment (our proposals, acknowledged) and one for the remote (their proposals, acknowledged).

   (Note that an implementation MAY optimize this internally, for
   example, pre-applying the changesets in some cases).

In our case, we apply the unacked changes immediately into
staging_cstate, and save them in an unacked_changes array.  That array
gets applied to staging_cstate as soon as it's acked (we only allow
one outstanding update_commit, so we only need one array).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell