Mbq.metrics

Changes the way metrics from the middleware are sent to Datadog. Before they had namespaces like `<servicename>.request`, so that each service needed a separate monitor. The metrics namespaces are now`mbq.metrics.connections`, `mbq.metrics.request_duration_ms`, and `mbq.metrics.connections`. This way all services using mbq.metrics can share the same Datadog monitors for request metrics. This is consistent with how we do monitors for mbq.pubsub, mbq.ranch, and mbq.atomiq.

This change also makes a breaking change to the `init` function, which now requires a `service` argument, as well as an `Environment` object from `mbq.env`.

* Drop support for Python 2.7
* Update dogstatsd to 0.24.0

Note: only doing a minor version bump because we don't have any services using mbq.metrics with Python 2.7.

* Internal: use a global statd instance
* mbq.metrics is only usable on Linux (including tests). This has always been true but now the tests won't run on macOS so a Docker-based test setup is provided in addition to Travis CI.
* isort

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