chainntnfs/bitcoindnotify: filter out mempool spends from relevant txs
chainntnfs/bitcoindnotify: filter out mempool spends from relevant txs

This commit fix a bug within the bitcoind notifier logic, which would
ignore the passed mempool argument, and notify spentness whether the
spending transaction was confirmed or not. The logic used to fix this is
similar to what is already done for the btcd backend.

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