chainntnfs/txnotifier: refactor ProcessRelevantSpendTx to detect script spends
chainntnfs/txnotifier: refactor ProcessRelevantSpendTx to detect script spends

In this commit, we refactor the TxNotifier's ProcessRelevantSpendTx to
also detect script spends. This can easily be done as the transaction
filtering logic was refactored into its own method in a previous commit.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd