[#324] morph/listener: Register handlers of new chain blocks

Extend Listener with RegisterBlockHandler method. All block handlers are
called on each block read from Subscriber.BlockNotifications channel.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/morph/event/handler.go

@@ -1,8 +1,15 @@
 package event
 
+import (
+	"github.com/nspcc-dev/neo-go/pkg/core/block"
+)
+
 // Handler is an Event processing function.
 type Handler func(Event)
 
+// BlockHandler is a chain block processing function.
+type BlockHandler func(*block.Block)
+
 // HandlerInfo is a structure that groups
 // the parameters of the handler of particular
 // contract event.