invoices: refactor invoice subscriptions to avoid blocking

This commit fixes a potential blocking when notifying invoice updates.
When a new subscription client is created followed by an immediate
cancel, it's likely the client will be removed from the registry's
map(noop) and then added to its map again. This subscription will then
be kept in registry until lnd is restarted. Another more serious issue
is when multiple subscriptions are made for the same invoice, when the
above case happens, other subscriptions may never send invoice updates
because a previous client has a stopped notification queue that blocks
following notifications.

lnrpc/invoicesrpc/invoices_server.go

@@ -244,6 +244,8 @@ func (s *Server) SubscribeSingleInvoice(req *SubscribeSingleInvoiceRequest,
 	}
 	defer invoiceClient.Cancel()
 
+	log.Debugf("Created new single invoice(pay_hash=%v) subscription", hash)
+
 	for {
 		select {
 		case newInvoice := <-invoiceClient.Updates:
@@ -265,7 +267,9 @@ func (s *Server) SubscribeSingleInvoice(req *SubscribeSingleInvoiceRequest,
 			}
 
 		case <-updateStream.Context().Done():
-			return updateStream.Context().Err()
+			return fmt.Errorf("subscription for "+
+				"invoice(pay_hash=%v): %w", hash,
+				updateStream.Context().Err())
 
 		case <-s.quit:
 			return nil