feat: add ramp up feature to load test client

[alastairtree]

Hi Tore

Thanks for a great library. Are you interested in adopting this small addition to the console client to allow you to ramp up the number of threads over time? I find it really useful for identifying the optimum number of threads before increasing them fails to increase the number of req/sec the API under test handle.

This change:

• add a ramp up option to console app "-r" which will repeat the test a number of time
• it will keep track of the best perf based on number of requests per seconds achieved
• it increase the number of threads by one if requests per sec has increased
• it will not increase threads if the last iteration caused an increase in the number of errors
• reports the optimum number of threads completion to the user
• it will abandon ramping up after 3 attempts if doing so has lead to no increase in req/sec or to more errors

Thanks

Alastair