Handling for sign_counterparty_commitment failing during normal op

Handling for sign_counterparty_commitment failing during normal op

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

Here we add initial handling of sign_counterparty_commitment
failing during normal channel operation, setting a new flag in
`ChannelContext` which indicates we should retry sending the
commitment update later. We don't yet add any ability to do that
retry.