We don't have any user-generated signals to handle by shutting down in
an app.
One less thing to worry about. Now it's just the global
TerminationFlag that is problematic when the code runs in just one
process.
Pull the notification pieces out of SigUtil.cpp - not signal safe,
and invoked only from LOOLWSD anyway.
In a non-blocking world, the socket close notification sends are
instant - so more work required to count-down / timeout remaining
clients.
