Support RBF

[kayabaNerve]

Beyond needing a dedicated secondary signing loop, for re-signing a TX which has taken too long, we need to update the scheduler.

serai/processor/src/main.rs

Lines 221 to 227 in 0c9c1ae

	for branch in branches {
	substrate_mutable
	.schedulers
	.get_mut(key.as_ref())
	.expect("didn't have a scheduler for a key we have a plan for")
	.created_output(branch.expected, branch.actual);
	}

This is the problematic block.

[kayabaNerve]

I had an interesting idea where instead of supporting RBF, we create an anyone-can-spend output on each transaction. If it lags, anyone could use child-pays-for-parent to bump its priority.

Unfortunately, this doesn't work as a malicious actor could spend the output without RBF enabled. If they don't increase the fee rate, this wouldn't be an instance of CPFP, yet an honest volunteer wouldn't be able to perform CPFP due to the occupation of the output.

[kayabaNerve]

mempoolfullrbf solves the above concern. If it's truly enabled by default now, it'd be the solution here.

[kayabaNerve]

Resolved in above commit on the processor-smash branch.

[kayabaNerve]

https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-28.0.md notes how mempoolfullrbf was enabled by default.