change the callable functions in the mod_watchdog API

to optional hooks to avoid module ordering or other symbol
resolution issues; affected:

  ap_watchdog_get_instance
  ap_watchdog_register_callback
  ap_watchdog_set_callback_interval


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@820427 13f79535-47bb-0310-9956-ffa450edef68