Handle monitor update failure during funding on the fundee side

Handle monitor update failure during funding on the fundee side

This carries a surprising amount of complexity despite only being
possible in the case where monitor updating failed during the
processing of funding_generated. Specifically, this requires
handling rebroadcasting funding_locked once we successfully persist
our monitor again.

As an alternative we could never send funding_signed when the
monitor failed to persist, but this approach avoids needless
delays during funding.