chaintnfs/btcdnotify: make cancellation of block epoch synchronous

This commit is meant to fix an occasional flake in the interrogation
tests cause by the async nature of the cancellation of block epoch
notifications. This commit modifies the cancellation to now be fully
synchronous which should eliminate this flake.

chainntnfs/btcdnotify/btcd.go

@@ -262,6 +262,8 @@ out:
 
 				close(b.blockEpochClients[msg.epochID])
 				delete(b.blockEpochClients, msg.epochID)
+
+				close(msg.done)
 			}
 		case registerMsg := <-b.notificationRegistry:
 			switch msg := registerMsg.(type) {
@@ -707,6 +709,8 @@ type blockEpochRegistration struct {
 // cancel an outstanding epoch notification that has yet to be dispatched.
 type epochCancel struct {
 	epochID uint64
+
+	done chan struct{}
 }
 
 // RegisterBlockEpochNtfn returns a BlockEpochEvent which subscribes the
@@ -726,10 +730,17 @@ func (b *BtcdNotifier) RegisterBlockEpochNtfn() (*chainntnfs.BlockEpochEvent, er
 		return &chainntnfs.BlockEpochEvent{
 			Epochs: registration.epochChan,
 			Cancel: func() {
-				select {
-				case b.notificationCancels <- &epochCancel{
+				cancel := &epochCancel{
 					epochID: registration.epochID,
-				}:
+					done:    make(chan struct{}),
+				}
+
+				select {
+				case b.notificationCancels <- cancel:
+					select {
+					case <-cancel.done:
+					case <-b.quit:
+					}
 				case <-b.quit:
 					return
 				}