✨ Discovery retry support + fixed a race condition in network handling

[ninadbstack]

General snapshot discovery retry mechanism
Context:

• In some cases a page fails to load momentarily, sometimes due to network failure or sometimes due to an unforeseen error, or resource crunch etc
• In such conditions so far we reported error and dropped that snapshot - which leads to a missing snapshot warning on percy. For some customers this was more common than others.

Changes:

• We now give an option under discovery section of config to retry discovery. For now its false by default
• In case the retry is set to true we attempt discovery upto 3 times, printing a log line whenever its retrying the snapshot
• We can also use this retry option on per snapshot basis by passing {discovery: {retry: true}} in per snapshot options.

Race condition in discovery for certain cases:
Context:

• Sometimes browser requests same url multiple times in parallel. In such cases we record both as separate requests under #requests but we only get loadingFinished event for one of them. -> other request remains stuck and we throw idle timeout.
• While debugging i did find that the request is infact stuck in browser - we see it in pending state. But for these requests we in general do get responseReceived callback, but dont get loadingFinished.
• We need to debug further why the request is getting stuck, doesnt look like its stuck on cli.

Changes:

• We are now maintaining a finishedUrls set. Whenever we look for idle we now ignore all the urls which are successfully finished at least once before.
• This is okay to do as in resources we can only attach one response per url and so we dont need to have multiple response objects.

[prklm10]

LGTM 🍍