Service: add a conflict? function.

Two services can conflict with one another. Now we can detect that.
Restructure core to support instrumentation service.

Cores now have a conj-service function to add services. Conj-service
checks for conflicts, preventing you from adding multiple incompatible
services to the same core. This allows (config/instrumentation) to
*replace* any existing instrumentation service, while simultaneously
supporting preserved services across core reloads.

Cores now come with a default instrumentation service.

Also updates and improves several tests for riemann.config.

Also introduces faster restarts of thread-services by interrupting the
thread, which makes long-polling thread-services feasible.