Added the Daemon and MutlithreadedDaemon classes. The stager, tape gateway,
tape bridge and vdqm daemons will eventaully inherit from one of these new classes. This commit is a step towards this goal. Please note that this commit modifies the castor::rh::Server and castor::server::BaseDaemon classes in not so desirable ways. The castor::rh::Server has been modified so that it does not use the MetricCollector class. The castor::server::BaseDaemon class has been modified so that it does not use either the UDPListenerThreadPool or the MetricCollector class. I will put back this code in another commit.