Bring skydebugger closer to clean-shutdown

This teaches the SkyDebugger prompt how to tell
the sky debugger (server) to shut down instead of
just calling exit(0).

This also teaches the WindowManagerApp (server) how
to tear down all of its connections itself instead
of depending on the pipes to do so (which would
crash when youd delete the WindowManagerApp as the
pipes could outlive it with WindowManagerImpl objects
containing raw pointers back to the WindowManagerApp).

Shutdown is not yet clean.  It errors out trying to
talk to the X11 server, but it's closer to clean
than it was prior to this change.  I may add back
and exit(0) to side-step shutdown until it can be
made fully clean.

R=jamesr@chromium.org, sky@chromium.org
BUG=430291, 430242

Review URL: https://codereview.chromium.org/695183003