Context objects now use a contextmanager (and update gmg shell to use it)

Context objects now use a contextmanager (and update gmg shell to use it)

This means that we have a really convenient way to make sure that you
have a context/request that threads its way through the application,
where everything needed gets "shut down" appropriately by the end.

You always get a context object via a context manager!  And by the time
you're done with it, things should be cleaned up.