Coordinator keeps internal mempool state

[f321x]

The coordinator currently implements its own mempool to lookup the bond inputs for malicious spends. This increases memory usage and seems inefficient. Luckily this is not necessary anymore as Bitcoin Core implements the RPC call gettxspendingprevout since v25.0.0 which offers exactly this functionality. Unfortunately the bitcoincore_rpc rust crate used in the coordinator doesn't offer this API call yet. It would be possible to implement a custom API call, or even better, do an upstream PR to bitcoincore_rpc to implement this API call.