Implement Retry Mechanism for HttpExtractor

[georg-schwarz]

Closes #403.

Example

block CarsExtractor oftype HttpExtractor {
		url: "https://gist.githubusercontent.com/noamross/e5dasdasd..."; // invalid URL
		retries: 3;
		retryBackoff: 10000;
	}

[rhazn]

I think our default strategy should be exponential backoffs and we should enforce some form of minimal delay. Right now it is trivial to missconfigure (or deliberately configure in the case of JV cloud) this to spam a source with 1ms delayed requests.

I'd go with exponential backoff and minimal delay of 2s. So we'd have 2, 4, 8... etc backoffs.

[georg-schwarz]

• Added a prop retryBackoffStrategy with allowed values exponential (default) and linear.
• Renamed retryBackoff to retryBackoffMilliseconds
• Prop retryBackoffMilliseconds must be 1000 or larger; edit: 2000ms as minimum

[rhazn]

It is wrong but intuitively, I'd assume exp backoff to be 1, 2, 4... for 1000ms. Maybe we force a min value of 2000 or do the 1,2,4 by hand?

I am fine with merging it like this as well though.