Allow wrk2 to start without specifying the rate parameter

[ol-damirm]

This solution will make it possible to run wrk2 as well as wrk. If the rate paremeter is omitted, then UINT64_MAX value will be used.

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Testing done:

# run wrk2 with rate parameter:
./wrk --connections 100 --threads 2 --duration 60s <url> --rate 500000
Running 1m test @ <url here>
  2 threads and 100 connections with desired rate 500000
...

# run wrk2 without rate parameter:
./wrk --connections 100 --threads 2 --duration 60s <url>
Running 1m test @ <url here>
  2 threads and 100 connections
...