Start using mozsvc's logging and metrics infrastructure

Necessitated some invasive changes to be really happy with mozsvc.
You could use something like:
```
from mozsvc.metrics import annotate_request
annotate_request(request, "shavar.downloads.unknown.format", 1)
```
And the count will be included in the request summary log.

But we should probably work on a more purposeful API for this in mozsvc.