The MixGraph workload should not be used with more than one thread

I spoke with the author to confirm that db_bench --benchmarks=mixgraph should only be used with --threads=1

The short term fix is to fail with an error if --threads is greater than 1.

From the author:
For the first question, I have tried the multi-thread, it works. Specify the thread number in db_bench options. But there is a problem. Each thread will generate the workload with the expected distribution. So the actual workload shape will change (add together).

A proper fix might be to have a background thread generate the workload and place requests on queues processed by user (foreground) threads that will perform RocksDB API calls.