@request_tokens and @requests appears unused

[dyladan]

To be honest I'm not sure if this is a documentation issue, a bug, or just tech debt left over.

The instance variable @request_tokens is documented as a way to control the number of open requests, but I can't see anywhere in the plugin code or the HTTP mixin code that actually uses it for anything. The commit (899196f) that authored this variable does appear to actually have used it at one point (899196f#diff-ade3d8629809fb4e534347d2d6fb03b65b0b0d852fcce1ef3c9fa4090cda1562R101).

Similarly, the instance variable @requests is also apparently unused. It also appears to have been used in the commit that originally authored it (287dbf5).

I am using this plugin as an example for another plugin that I'm developing, but this part really confused me. Are these variables actually used in some hidden way, or are they just left over from a previous version of the plugin? If they're left over tech debt I'm happy to make a PR to remove them so others aren't confused in the future.