[#1546] morph/event: Merge notification parser and handlers

They are decoupled, but it is an error to have a handler without a
corresponding parser. Register them together on the code level and get
rid of unreachable code.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/morph/event/handlers.go

@@ -18,17 +18,23 @@ type BlockHandler func(context.Context, *block.Block)
 type NotificationHandlerInfo struct {
 	scriptHashWithType
 
-	h Handler
+	parser   NotificationParser
+	handlers []Handler
 }
 
-// SetHandler is an event handler setter.
-func (s *NotificationHandlerInfo) SetHandler(v Handler) {
-	s.h = v
+// SetParser is an event handler setter.
+func (s *NotificationHandlerInfo) SetParser(p NotificationParser) {
+	s.parser = p
+}
+
+// AddHandler adds an event handler.
+func (s *NotificationHandlerInfo) AddHandler(v Handler) {
+	s.handlers = append(s.handlers, v)
 }
 
 // Handler returns an event handler.
-func (s NotificationHandlerInfo) Handler() Handler {
-	return s.h
+func (s NotificationHandlerInfo) Handlers() []Handler {
+	return s.handlers
 }
 
 // NotaryHandlerInfo is a structure that groups