Comparing performance/quality trade-off of SPEC and VTR random number generators.

[soheilshahrouz]

VTR currently uses a random number generator implemented in vtr_random.cpp file. There is another RNG implementation in specrand.cpp that is conditionally compiled. In #2793, Vaughn suggested replacing VTR's random number generator with SPEC RNG. To do this, we need to evaluate the performance and quality trade-offs between these two implementations.