Use unconfirmed coin for rbf fee

[pythcoiner]

Is there a reason we not allow the user to use unconfirmed coin for pay the fees of a rbf?

[jp1ac4]

There are some restrictions on which unconfirmed coins can be used in the replacement: #826 (comment).

We'll need to check the details again, but we may need to consider some additional unconfirmed coins when creating the replacement in case the transaction to be replaced is not in the mempool but there are some other conflicting transactions: #853 (comment).