Add stats-based random alloc/dealloc generation and simulation.

[guangli-dai]

This is a local stress test for jemalloc.

1. By calling generateAllocs.sh, a random alloc/dealloc file will be generated based on the given size distribution;
2. With the file generated in the first step set in randomTest.sh, the generated allocs/dallocs will be executed and corresponding jemalloc stats will be probed to better understand the influence of jemalloc-internal changes.