feat: report many pipeline resets

[feugy]

This is a follow up of #281
Now, request iterator resets when some of its setupRequest() returning falsey value (previously it the pipeline was simply crashing).

But this may actually obfuscate bugs of the tested service: often setupRequest() fails as the previous response did not contained expected data.

This PR simply displays how many times the pipeline reseted, so users could decide whether it is intended or not.

┌─────────┬────────┬────────┬────────┬────────┬───────────┬───────────┬────────────┐
│ Stat    │ 2.5%   │ 50%    │ 97.5%  │ 99%    │ Avg       │ Stdev     │ Max        │
├─────────┼────────┼────────┼────────┼────────┼───────────┼───────────┼────────────┤
│ Latency │ 121 ms │ 624 ms │ 890 ms │ 914 ms │ 541.06 ms │ 237.32 ms │ 1002.16 ms │
└─────────┴────────┴────────┴────────┴────────┴───────────┴───────────┴────────────┘
┌───────────┬────────┬────────┬────────┬────────┬────────┬─────────┬────────┐
│ Stat      │ 1%     │ 2.5%   │ 50%    │ 97.5%  │ Avg    │ Stdev   │ Min    │
├───────────┼────────┼────────┼────────┼────────┼────────┼─────────┼────────┤
│ Req/Sec   │ 218    │ 218    │ 372    │ 479    │ 356.5  │ 70.29   │ 218    │
├───────────┼────────┼────────┼────────┼────────┼────────┼─────────┼────────┤
│ Bytes/Sec │ 190 kB │ 190 kB │ 372 kB │ 465 kB │ 369 kB │ 67.3 kB │ 190 kB │
└───────────┴────────┴────────┴────────┴────────┴────────┴─────────┴────────┘

Req/Bytes counts sampled once per second.

10k requests in 28.09s, 10.3 MB read
18 errors (0 timeouts)
request pipeline was resetted 4 times

Note: I know test/progressTracker.test.stub.js isn't part of the test suite, but as I've changed it to troubleshoot the feature, I though it would be fine to keep a new test in it.
The feature is properly covered in est/httpClient.test.js and test/run.test.js test files.

Edit: this PR actually unveiled some flaws in the previous implementation.
Reseting request pipeline the way I did is wrong, as it may leave currentRequest.requestBuffer to null.

This new version fixes the flaw, and also handle a specific case where the very first setupRequest() fails. In this situation, we can not reset pipeline (or it'll never end), and we must exit on an error.

[mcollina]

lgtm