Move funding signing after commitment

[devrandom]

An external signer wants to see that the commitment transaction was signed by the counterparty before releasing the funding signature. Without the commitment counter-signature, the signer cannot be sure that the channel funds won't be locked forever.

Since the commitment tx depends on the funding tx, we need to separate funding generation from funding signing:

1. generate the funding transaction
2. generate the commitment transaction
3. have the commitment transaction signed by the counterparty
4. provide the signature to the signer via validate_holder_commitment
5. sign the funding transaction

This could be done with a new type of Event callback.

[TheBlueMatt]

CC @dunxen we should make sure we get this done as a side-effect of dual-funding.

[dunxen]

CC @dunxen we should make sure we get this done as a side-effect of dual-funding.

Thanks for tagging. Will make sure we do.