Remove managing tests for now, not handling processes it creates

These tests are not properly handling the processes they spawn
which causes processes to be left running. This can cause lots of
problems when running the test suite multiple times in a row.
For now, I'd rather remove these tests, and re-think how to
properly test this logic. The unit tests should provide decent
coverage, and the bench host can always be started to tests the
full stack.
Allow configuring a `runner` per host

This allows hosts to configure their own `runner` for handling
service handlers. Sanford provides a `Runner` mixin for defining
custom runners. This provides the required interface and hooks
for adding custom logic.

This allows extensions of Sanford to insert a custom `Runner`, for
use in the normal processing of requests. It also provides the
ability to use a different `Runner` when manually running a service
handler (like in tests) than what is used when a server receives a
request. This was already being done by the `TestRunner` but is now
a more formalized concept. Overall, this provides a more control
over running service handlers in Sanford's stack.