tpcc: investigate percona tpc-c load generator

[rjnn]

Percona has a TPC-C load generator which we should take a close look at to see if its implementation is doing anything that we might want to also do, with respect to schemas, indexes, and query rewrites. The SIGMOD 2017 Amazon Aurora paper uses this load generator for benchmarking Aurora's TPC-C performance.

cc @jordanlewis