REFACTOR: Shutdown process and call event outside

- Add explicit mprDestroyEventService to destroy all dispatchers
- Add options to mprCreateEventOutside to support blocking flags
- Change mprTerminate to only set stopping flag. Defer actual shutdown to mprDestroy
- Remove mprIsExiting definition
- Change mprPauseGC to return boolean if MPR is being shutdown and GC cannot be paused
- Rename MPR_FINISHED => MPR_DESTROYED, MPR_STOPPING_CORE => MPR_DESTROYING