[core] rate limit handling on online file source

[ivovandongen]

Fixes #5821

[ivovandongen]

I've implemented the core and platform specifics. All platforms, expect for Android, have the same unit test. It would be nice to also test this against an actual server with a low rate limit as well, just to be sure.

• @jfirebaugh Can you take a look at the core stuff?
• @tmpsantos Same for linux/QT?
• @frederoni iOS/macOS?
• @zugaldia Android?

[jfirebaugh]

Can you move this to http_header.hpp and make the signature:

optional<Timestamp> parseRetryHeaders(const optional<std::string>& retryAfter,
                                      const optional<std::string>& xRateLimitReset);

Advantages:

• No need for RetryAfter type
• Factor conditional and integer conversion logic from individual SDK implementations

[ivovandongen]

@jfirebaugh Thanks for taking a look. I've changed the implementations and will rework the tests.

[ivovandongen]

@jfirebaugh

• I've extracted the timeout functions to a header in src/util so it is testable, didn't want to add it to the public header as it is not quite public api. Let me know if this isn't what you had in mind.
• I kept the integration tests so that the platform bindings are tested as well.
• I've added unit tests on the timeout functions directly.

When writing the unit tests I ran into a little issue where the the timeout became negative on high number of errors as the sign bit flipped (here among others). Changed all three exponential backoffs so that doesn't happen (1 -> 1u) here

[jfirebaugh]

Prefer early returns to temporary variables:

if (retryAfter) {
    try {
        return ...;
    } catch ... {
        return ...;
    }
}

if (xRateLimitReset) {
    return ...;
}

return {};

[tmpsantos]

nit, missing space after if

[tmpsantos]

Thanks for adding more tests!

[ivovandongen]

@tmpsantos Thanks for the review! Made the changes as suggested. Waiting on CI and then I'll merge.