Find way to drop RBF'd `Pending` payments

[tnull]

In #432 we introduced tracking of onchain payments, which are marked as PaymentStatus::Pending until reaching ANTI_REORG_DELAY confirmations.

However, that means that if a previously tracked payment get's RBF'd the original might be marked as Pending forever in our store. We should find a way to detect double-spends and eventually drop the original entry.

This should possibly happen in conjunction with #367.

[tnull]

Once we do this we could consider also updating the store after updating with mempool transactions, i.e., in apply_unconfirmed_txs.