push threadlocals while executing config.include functions

[mmerickel]

We don't push threadlocals in the main because there is no clear contract on its lifecycle. However we can push them during config.include(...) calls so that they are available there. Just a little more incentive to put all your code into include functions!

[piotr-dobrogost]

We don't push threadlocals in the main because there is no clear contract on its lifecycle.

Michael, could you please elaborate on this lifecycle issue (I'm curious)?
Btw, is there any section in the docs for this kind of information?

[mmerickel]

The lifecycle issue is that there is no clear ending point to the Configurator lifetime. This means that it cannot automatically push the threadlocals because it cannot remove them at an appropriate spot. The only possible spot is make_wsgi_app but that is not good enough because the Configurator can be used to make multiple wsgi apps in testing scenarios and such.

If you combine this PR with #2873 and the pending #2874 we can rewrite example code as below and have the entire config-time covered by threadlocals.

with Configurator(settings=settings) as config:
    config.include(...)
    config.add_view(...)
    return config.make_wsgi_app()