chain/neutrino: fix race in NotifyBlock

Currently NotifyBlock releases the clientMtx before calling a public
version of NotifyReceived that reacquires clientMtx. This can have
unexpected behavior because the value of isScanning() could change
between lock acquisitions. We switch to using the internal
notifyReceived so that our read on isScanning() is consistent for the
duration of the call.

chain/neutrino.go

@@ -443,13 +443,13 @@ func (s *NeutrinoClient) Rescan(startHash *chainhash.Hash, addrs []btcutil.Addre
 // NotifyBlocks replicates the RPC client's NotifyBlocks command.
 func (s *NeutrinoClient) NotifyBlocks() error {
 	s.clientMtx.Lock()
+	defer s.clientMtx.Unlock()
+
 	// If we're scanning, we're already notifying on blocks. Otherwise,
 	// start a rescan without watching any addresses.
 	if !s.isScanning() {
-		s.clientMtx.Unlock()
-		return s.NotifyReceived([]btcutil.Address{})
+		return s.notifyReceived([]btcutil.Address{})
 	}
-	s.clientMtx.Unlock()
 	return nil
 }