Investigate fixes for BTCPay unfulfilled invoice spam

[nochiel]

Invoice spam was reported as a problem here: https://github.com/orgs/BlockchainCommons/projects/3#card-86416438

• Verify that this been fixed by:
  • Bug allowing Spam/Denial of Service, due to gap limits in other wallets (& related feature requests to fix) btcpayserver/btcpayserver#3190
  • Bug allowing Spam/Denial of Service, due to gap limits in other wallets (& related feature requests to fix) btcpayserver/btcpayserver#3190