Improve performance of workload generator

[ngcgarcia]

[blocked on #50]

Issue

#50 made the workload generator multithreaded. However, this PR did not fine-tune the generator to make it as effective as possible. There are many different strategies to investigate for improving performance:

• Using non-blocking stubs (see: Make workload generator multithreaded #50 (comment))
• Using separate channels per worker (see: Make workload generator multithreaded #50 (comment))
• Changing the ThreadPoolExecutor or queue.
  • Creating a new handler (ThreadPoolExecutor documentation under rejected tasks) so that failed executor.submits block and retry (see: Make workload generator multithreaded #50 (comment))
  • Removing the queue or adjusting the max queue size (see: Make workload generator multithreaded #50 (comment))