chainntnfs: Fix btcdnotify dispatch race condition
chainntnfs: Fix btcdnotify dispatch race condition

This race condition can occur if a transaction is included in a block
right when a notification is being added to the notifier for it AND when
the confirmation requires > 1 confirmations. In this case, the
confirmation gets added to the confirmation heap twice.

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