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I.e. LD_PRELOAD style.

Could potentially allow us to do some crazy things, like make LevelDB operations ULT-able, and open up the use of benchmarks like fio and mdtest. We'd need some process-global setters to "install" pools to direct IO to and perform other management functionality. Of course, usage scenarios would be restricted (mixing pthreads and ABT ULTs = BAD).