Tie up maker's UTXOs in the mempool as a DOS method

[chris-belcher]

1. Attacker is a taker and coinjoins with many makers and sets a 0.5 sat/b miner fee on the transaction, which remains unmined for a long time

2. Makers are by-default programmed to not offer UTXOs that are currently in the mempool

3. The attacker could then run their own makers, either for sybil-attack/deanonymizing or to increase their own maker income due to reduced supply

Thanks to a redditor who PM'd me this idea.

Possible solutions are:

1. Proper handling of miner fees as in Make use of maker contribution to tx fee #120 (comment) where every make has a "minimum fee rate" so they can use that to stop their UTXOs being stuck with a very low fee.

2. Enable RBF on everything, and potentially still offer maker's UTXOs for coinjoining if the taker pays for the RBFing.

For now we yield-generators should monitor how many UTXOs and transactions we have tied up in the mempool, and raise the alarm if it looks like somebody is actually doing this.