Github search has custom rate limits

[paulmelnikow]

I ran into some unexpected behavior locally which we're also seeing in CI. The Github service tests are flaky, and they sometimes pass or fail depending on the order in which they are run.

Our token logic doesn't correctly distinguish between the standard 12,500 requests per minute, and the 30 requests per minute provided by the Search API. Once a token is used for a search request, the server thinks it's exhausted and stops using it.

This is pretty serious: it means in production we're recycling tokens very prematurely.

I'd argue our github-auth middleware deserves some separate unit tests or integration tests.

This is independent of #979.

[paulmelnikow]

Have started refactoring and working on this.

master...paulmelnikow:github-token-pool

[paulmelnikow]

After a massive amount of incremental work, this is in the home stretch: #1205.

[paulmelnikow]

This was fixed in #1205.