maintner: use GitHub rate limits for throttling requests
maintner: use GitHub rate limits for throttling requests

The HTTP response object contains an indication of how much (or until
when) to wait before the next successful API call.

This makes the code use this information to obey the rate limit
constraints.

Change-Id: I22cf851220db6f361f8663be28d49179f52dd623
Reviewed-on: https://go-review.googlesource.com/c/build/+/170959
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build