Preparing for a deployment of worker MPM

We are seeing regular connection flooding in the east coast morning
time. Because mirrorbrain needs to maintain backend postgres
connections, we cannot just increase the prefork apache process counts
any more. So as a means to cope with the problem, attempting to switch
to worker MPM.
Merge pull request #64 from jenkinsci/mpm-worker

[INFRA-96] Preparing for a deployment of worker MPM