send a signature instead of a private key for cooperative closes

It seems odd at first that the `coop_close` message (sent by the swap taker) contains a private key.

This *should* be safe because the swap taker must create a random private key for each swap (not derived from an HD wallet). This simplifies the protocol because the swap maker can create the `claim_by_coop` transaction immediately without further communication with the swap taker.

An alternative approach would be for the swap taker to send a signature signed with `SIGHASH_NONE | ANYONECANPAY`. With this approach, the swap taker would not have to create a random private key for each swap, but could safely use an xpub scheme to derive their key. This could simplify backups because they do not need to synchronously backup a random private key for each swap; a derived key can be restored by iterating a predefined xpub branch. On the other hand, if the taker loses the `` or `` of a swap, it won't matter if they can recover their private key. The taker must always back up some information for each swap.