Concurrency stress test

[g105b]

This PR includes two new scripts: a dummp HTTP server and concurrency tester.

This isn't real-life HTTP testing, but more to show that the library can handle hundreds of concurrently downloading requests without breaking.

closes #7

Example output:

Target: http://127.0.0.1:8080/?type=big-page&size-kb=512&delay-ms=25&speed-kbps=2048
Ramping concurrency from 10 to 1000 in steps of 10

Concurrency   10 | ok   10 | errors    0 | 9.722s | 1.03 req/s | memory   18 MiB
  Statuses: 200=10 
Concurrency   20 | ok   20 | errors    0 | 6.401s | 3.12 req/s | memory   46 MiB
  Statuses: 200=20 
Concurrency   30 | ok   30 | errors    0 | 9.452s | 3.17 req/s | memory   90 MiB
  Statuses: 200=30 
Concurrency   40 | ok   40 | errors    0 | 12.559s | 3.19 req/s | memory  144 MiB
  Statuses: 200=40 
Concurrency   50 | ok   50 | errors    0 | 15.628s | 3.20 req/s | memory  214 MiB
  Statuses: 200=50 
Concurrency   60 | ok   60 | errors    0 | 18.837s | 3.19 req/s | memory  294 MiB
  Statuses: 200=60 
Concurrency   70 | ok   70 | errors    0 | 21.921s | 3.19 req/s | memory  102 MiB
  Statuses: 200=70 
Concurrency   80 | ok   80 | errors    0 | 24.957s | 3.21 req/s | memory  216 MiB
  Statuses: 200=80