make the configurator a context manager

[mmerickel]

There's some subtle bugs in apps that hit certain APIs during configuration because parts of pyramid rely on threadlocal state that may or may not be available.

An example:

config = Configurator()
config.override_asset(to_override='myapp:locale/', override_with='yourapp:locale/')
config.add_translation_dirs('myapp:locale')

The result here is that the translations are still using myapp:locale because add_translation_dirs resolves those asset paths at config-time. Asset overrides work by using custom pkg_resources hooks that only affect pkg_resources when a registry is pushed on the threadlocal stack. Normally this would require surrounding relevant code with config.begin() and config.end(). We've done a poor job of documenting this.

I propose an optional context manager that we can implement easily (we already did something like this for p.paster.bootstrap in #2760) in order to manage the lifecycle.

with Configurator() as config:
    config.override_asset(to_override='myapp:locale/', override_with='yourapp:locale/')
    config.add_translation_dirs('myapp:locale')

related: #2760 #2046

[mmerickel]

The particular example here was fixed by #2873 for the record.