Update to use new simplified mozsvc.metrics infrastructure

This updates syncserver to match the newly-simplified metrics-logging infra introduced in https://bugzilla.mozilla.org/show_bug.cgi?id=1012509

Basically, we replace "metlog" with stdlib logging.  There's some new JSON-based logging designed for heka in production, but it's off by default so we don't have to worry about it here.

@callahad r?