TEST: Add unit tests for mprStealSocketHandle, mprCloneSocket.
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.