REFACTOR: application shutdown

    Upgrade mpr pak.
    Refactor shutdown and mprCreateEventOutside
    Make mprShutdown (nee mprTerminate) non-destructive
    Implement hard guarantees with respect to graceful shutdown timeouts.
    Add mprCancelShutdown
    Invoke terminators twice to notify of shutdown.
    Test mprCreateEventOutside with new unit tests
    Expand doc for mprDestroy, mprShutdown, mprCancelShutdown etc.