Performance improvements for the circuitbreaker

We could improve the impact of the circuitbreaker by moving the stat-check outside of the request.

We could run the check in a single separate process per host, this process checks the accessibility (ForkedStorageCheck) once every x seconds and writes it's results in redis. When that result in redis is negative we perform the check again in the request, otherwise the circuitbreaker just allows access.

/cc @andrewn