Check unreachable task cache for task ID collisions on launch

As discussed in MESOS-6785, it is possible to crash the master by launching a task that reuses the ID of an unreachable/partitioned task. A complete solution to this problem will be quite involved, but an incremental improvement is easy: when we see a task launch operation, reject the launch attempt if the task ID collides with an ID in the per-framework unreachableTasks cache. This doesn't catch all situations in which IDs are reused, but it is better than nothing.