CiviEventDispatcher - Fix pass-by-reference of hook-style arguments for service-based...

CiviEventDispatcher - Fix pass-by-reference of hook-style arguments for service-based listeners

Suppose you are firing `hook_civicrm_foo` to a serivce-based listener
taht uses hook-style arguments. Conceptually, this is a call like:

Civi::service('foo')->hook_civicrm_foo($arg1, &$arg2, $arg3);

Before this patch, all values are pass-by-value. Changes to `&$arg2`
are not propagated back out.

The patch ensures that `&$arg2` propagates back out.