contractcourt: properly bump justice transactions

[morehouse]

Currently justice transactions use a confirmation target of 2, which should usually get them confirmed quickly. But if we get unlucky and mempool fee rates rise after we've created the justice transactions, the justice transactions can get stuck in the mempool for a long time, potentially long enough for the counterparty to claim their channel balance and escape justice.

Implementation

Ideally we'd be able to leverage the new sweeper to automatically bump fees every block. The problem is that we do need to maintain the custom logic to split the justice transaction into pinnable (i.e. HTLC claims) and non-pinnable (i.e. balance claim) groups after a few blocks delay, and we need to make sure that the non-pinnable group doesn't get any pinnable outputs from other channels grouped with it.

I think @yyforyongyu was working on "sweeper groups" at some point, which seems useful here to prevent aggregation.