Extend BumpFee to take into account an unconfirmed parent

[joostjager]

Currently the BumpFee rpc can be used to bump the fee of an unconfirmed transaction through CPFP.

It is however not straight-forward to do this because the caller needs to specify a fee rate that takes into account all unconfirmed parents of the sweep transaction. This seems to be impossible for the caller, because they don't know how their sweep request is going to be grouped with other inputs that may or may not have unconfirmed parents. Even the weight of the sweep transaction itself isn't known to the caller in advance.

It would be better if BumpFee detects that the input to sweep is still unconfirmed (or even has a chain of unconfirmed parents) and submits it to the sweeper subsystem with the appropriate parent info set. The sweeper already contains logic to take those unconfirmed parents into account during fee calculation.

In case the input transaction confirms on its own, it is probably desirable that the sweeper is aware of this. There is no need anymore to pay for the parent and the absolute fee on the sweep tx can be decreased.

[AndySchroder]

Seems like it would be good to have an opt in option to use your proposed (more intelligent behavior). This way backward compatibility is preserved and it is clearer when reading documentation what is actually done so you have to make an intentional choice if you want the actual fee to consider the effective fee from unconfirmed parents.

[AndySchroder]

One point to note: As a user with the current behavior, after making your first CPFP at a low effective fee rate, you can just keep trying to bump again without any loss (but time) because the next CPFP will RBF the first (and eventually find the effective fee you really want).