prefix_random distribution from FAST 2020 paper doesn't actually use key prefix

The prefix_random distribution as described in Appendix A of https://www.usenix.org/system/files/fast20-cao_zhichao.pdf doesn't actually generate a distribution based on the keyrange parameters. If key_dist_a and key_dist_b are 0, then `use_random_modeling` gets set to true, and then `DistGetKeyID()` doesn't get called.

It seems the behavior was broken by https://github.com/facebook/rocksdb/pull/6795 which purported to fix the `all_random` workload but apparently broke this one.