Per-task budget high-resolution timers (hrtimers).

As a step towards implementing more complex budget tracking
method (ex. BWI, VXR, etc.), we need per-task budget trackers
because we may be required to drain budget from a task, even
while it is suspended or blocked.
This patch does:
1) Replaces the per-CPU hrtimers with per-task hrtimers.
2) Plugin architecture for different budget policies.

This patch creates three budget draining policies:
SIMPLE, SAWARE (suspension-aware), and SOBLIV (suspension-oblivious).
However, only SIMPLE is supported by this patch.

SIMPLE (default):
Budget drains while the task is scheduled. Budget is preserved
across self-suspensions (but not job completions, of course).
Only SIMPLE is supported in this patch. (Maintaining current Litmus
functionality.)

SAWARE:
Draining according to suspension-aware analysis. Budget should drain
whenever a task is among the top-m tasks in its cluster, where
m is the number of processors in said cluster. This draining should
happen whether or not the task is actually scheduled.

SOBLIV:
Draining according to suspension-oblivious analysis. Budget should
drain whenever the task is scheduled or suspended (but not due to
preemption). Exception: Draining should halt when we can prove that
the task is not among the top-m tasks blocked on the same lock (i.e.,
on the PQ in the OMLP-family locking protocols).

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