Mark CallbackHandler as deprecated

- Deprecating starting with 2.7.4, as zend-eventmanager no longer
  requires it with v3.

Author: weierophinney

src/CallbackHandler.php

@@ -17,6 +17,11 @@
  * A handler for an event, event, filterchain, etc. Abstracts PHP callbacks,
  * primarily to allow for lazy-loading and ensuring availability of default
  * arguments (currying).
+ *
+ * This was primarily used in zend-eventmanager for managing listeners; as that
+ * component removes its usage of this class for v3, it is deprecated.
+ *
+ * @deprecated as of v2.7.4.
  */
 class CallbackHandler
 {