Rename 'shutdown' close reason and tuples to 'stop'

The 'shutdown' atom has a specific meaning inside OTP. We are
instead going to use 'stop' which is pretty much the equivalent
of what we actually do. 'shutdown' is now reserved for future
special processes implementation.
Rename 'halt' to 'stop' for better consistency

Now everywhere in Cowboy when we want to stop something we return
a 'stop' tuple instead of one of the many choices depending on
context that we had before.

This particular change affects middlewares, sub protocols and
REST handlers which were using 'halt' to stop processing.