Get rid of the Unallocated thread context state.

Get rid of the Unallocated thread context state.
Basically merge it in with Halted.
Also had to get rid of a few other functions that
called ThreadContext::deallocate(), including:
 - InOrderCPU's setThreadRescheduleCondition.
 - ThreadContext::exit().  This function was there to avoid terminating
   simulation when one thread out of a multi-thread workload exits, but we
   need to find a better (non-cpu-centric) way.