Rescan fair-scheduler.xml every n seconds

Currently, the FairScheduler configuration file fair-scheduler.xml is loaded only on ResourceManager startup.

We need:

• a mechanism to rescan / reload-if-changed this, often-changing piece of RM configuration
• from classpath / configuration file:
  • configuration file wins over classpath
• rescan every n seconds, where:
  • n <= 0 means don't rescan
  • n > 0 means rescan every n seconds