Add a TODO for an important issue for making async mon updates safe

Add a TODO for an important issue for making async mon updates safe

If we receive a monitor event from a forwarded-to channel which
contains a preimage for an HTLC, we have to propogate that preimage
back to the forwarded-from channel monitor. However, once we have
that update, we're running in a relatively unsafe state - we have
the preimage in memory, but if we were to crash the forwarded-to
channel monitor will not regenerate the update with the preimage
for us. If we haven't managed to write the monitor update to the
forwarded-from channel by that point, we've lost the preimage, and,
thus, money!