Added option to set random warehouse mode for tpcc workers

Hi,

this change adds an optional `tpcc_random_warehouse` parameter to the config, which make the worker choose their warehouse randomly on each transaction.

This TPC-C worker behavior is a part of CH-BenCHmark specification and I've seen it used a couple of times in other TPC-C based benchmarks for in-memory DBMS. Since the CH-BenCHmark would not be complete without it, I would really like for this functionality to be merged. Please, give me feedback if you'd like to change something.

I also think there is a need for an updated documentation for OLTPBenchmark, since the one on http://oltpbenchmark.com weren't updated recently. I could draft one which would contain the recent changes and additions, if you'd tell me, were I should submit it.