Handle retrying sign_counterparty_commitment failures

If sign_counterparty_commitment fails (i.e. because the signer is
temporarily disconnected), this really indicates that we should
retry the message sending which required the signature later,
rather than force-closing the channel (which probably won't even
work if the signer is missing).

This commit adds initial retrying of failures, specifically
regenerating commitment updates, attempting to re-sign the
`CommitmentSigned` message, and sending it to our peers if we
succed.