Use Granian instead of uwsgi

[leplatrem]

Let's try Granian instead of uwsgi.

Functional tests should pass (if they do, then investigate why they don't on mozilla/remote-settings#1162)

[alexcottner]

I'm not sure if you were looking for performance improvements with this, but it looks like we'll be getting one. I know this is a big shift in how kinto works (scaling at the container vs scaling at the app) but here are my results anyways:

• Same database, bucket, and collection with 10 tiny records.
• Same stress test in jmeter, running 200 concurrent users in 100 loops, 0 second ramp up time.
• Only hitting one endpoint: v1/buckets/tb1/collections/tc1/records?_sort=-last_modified&_limit=200

With main and uwsgi locked to 1 process: 80 seconds, 2 errors produced.
With this branch: 53 seconds, no errors produced.
Bumping the test up to 295 users results in no errors and takes 68 seconds to complete.
Beyond 295, I start seeing errors.

This could be a 34% improvement for performance if you look at test run time, or a 47% improvement if you look at how many concurrent users it can handle.