Multi-threaded client programs

[kavon]

Due to the challenges with implementing client-side performance auditing ( #28 ), we're limiting this prototype to single-threaded client programs.

One way in the future to get around this without thread-local storage to hold an entry counter is to have one thread dedicated to randomly modifying a global variable that indicates whether version A or B should be used, with threads only reading that on each function entry. The added benefit is that all threads, at roughly the same time, will use the same version during each burst, so the I-cache is flushed at once for all.