Allow benchmark mode

[jcchavezs]

Right now go-ftw executes requests one by one and then pull logs expecting the output to so the assertion.

While this is good, sometimes you want to profile and/or measure performance with close to real test data and go-ftw is a good option for this.

I wish go-ftw includes an option where N requests can be sent concurrently. This would allow to collect profiling data to debug latencies when evaluating and triggering rules.