Replace junit-benchmarks with JMH

Replace junit-benchmark with JMH. This includes the following tasks:

• Evaluate which existing benchmarks should be kept and port them to JMH.
• Add an initial JMH benchmark for TINKERPOP-957
• See if we can trigger perf tests with the same scheme that is currently used

  mvn verify -DskipPerformanceTests=true/false

• Write developer docs that outline our initial performance testing approach.