github-proxy: Limit concurrent requests per token

Created by: ryanslade

Our GitHub proxy is in place to limit the number of concurrent API requests we make, currently set at 1. However, it appears that this concurrency limit is only really valid when all calls are made on behalf of the same token, see here: https://developer.github.com/v3/guides/best-practices-for-integrators/#dealing-with-abuse-rate-limits

Specifically: Make requests for a single user or client ID serially. Do not make requests for a single user or client ID concurrently.

We should alter the proxy so that it never sends concurrent requests for the same token, but does allow concurrent requests if they are for different tokens. This concurrency level should be configurable.

We'll need to update our src_github_rate_limit_remaining metric, but we probably don't want to increase the cardinality based on tokens.