Allow holder commitment and HTLC signature requests to fail

Allow holder commitment and HTLC signature requests to fail

As part of the ongoing async signer work, our holder signatures must
also be capable of being obtained asynchronously. We expose a new
`ChannelMonitor::signer_unblocked` method to retry pending onchain
claims by re-signing and rebroadcasting transactions. Unfortunately, we
cannot retry said claims without them being registered first, so if
we're not able to obtain the signature synchronously, we must return the
transaction as unsigned and ensure it is not broadcast.