Stirring performance, boys.

Fixed

• Greatly reduce CPU usage, particularly when receiving long-running or large responses. This was caused by a bug where timeout timers were not being cleared once they expired, effectively creating a repeating timer that would cause repeated extra polls. Huge thanks to @jacobmischka for finding and fixing this bug! (#348, #350)
• Return true for Body::is_empty for HEAD responses (#341, #343)
• Fix code coverage analysis failing to run in CI. (#351)