Refactored JobTracker into fixed/variable policies.

Review: https://reviews.apache.org/r/12924
Fixed bug where active TaskTrackers are killed.

Sometimes, TaskTrackers which are active will fail to get marked as such
because the scheduler is either a) too busy or b) has not received any
offers in a while.  This resolves the problem of TaskTrackers getting
killed by the launch timeout even though they are alive and well.

Furthermore, some parts of the scheduler don't need to be completely
synchronized because they don't modify shared data.
Allowed JobTracker to use unknown trackers.

The Mesos scheduler shouldn't discard task assignments for task trackers
that it doesn't know about.