Allow JobTracker to pause task scheduling

We want to have an ability to pause task scheduling in JobTracker.

The idea is: make job tracker still accept new jobs, but delay their running and do not schedule any new tasks from the currently running jobs.

It will help for example restarting the DFS cluster without affecting jobs: pause execution, restart the DFS, running tasks will fail, but will not be scheduled until the execution is resumed, so the job does not fail.

In general it should help fix non MR problems (DFS, network, etc.) while not failing running jobs and keep accepting new ones.

What do people think of the general idea?