Simplify configuration of test_lens.py.

We were reading test values for the received_date parameter, etc.  by parsing a
saved mime message, which didn't really make a ton of sense. Instead just get
test values by querying the database for a test message.
Don't call gevent.monkey.patch_all() in api/srv.py.

It breaks the api_client fixture in the tests in really weird ways, and isn't
actually needed. (We monkey-patch in bin/inbox-api, and gunicorn's GeventWorker
does it too: https://github.com/benoitc/gunicorn/blob/master/gunicorn/workers/ggevent.py#L170)