Due to the broadcast failure quite a few users are reporting channels
stuck in awaiting lockin. This commit adds a `dev-forget-channel`
command that checks whether the funding outpoint is in the UTXO, and
forgets the channel if not. The UTXO check can be overridden with the
`force` parameter, but that is dangerous.
Signed-off-by: Christian Decker <decker.christian@gmail.com>