Adding a test that exercises stask_wake.
Adding some systemtap probes.
Using a seq_cst store to be safe.
Fixing bug in joining.

First we should join all executors, THEN free them. This was causing problems
with the faster mutex (revealed by the faster mutex).
Merge branch 'task_proc_exec_refactor' of github.com:scottgw/quicksilver into task_proc_exec_refactor

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Merge branch 'faster_mutex' into task_proc_exec_refactor