bloomfile-test: use our own RNG for benchmark
bloomfile-test: use our own RNG for benchmark

Previously this benchmark used random(), which is slow and suffers from lock
contention. Now, we use a separate RNG per thread, which improves performance
of the benchmark and better focuses the results on the performance of the
underlying code.

Change-Id: I49f6163284ea2a63756ecdd4849b9828f0a3f3a7
Reviewed-on: http://gerrit.cloudera.org:8080/1272
Tested-by: Internal Jenkins
Reviewed-by: Jean-Daniel Cryans

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