[System] Fix build warnings
[Http]: Cleanup chained async operations; bug #19161.
[Http]: Cleanup and simplify the ServicePoint's connection group list.
Small fix.
[Http]: Only recycle ServicePoints from the idle timer (fixes #19823).

Thanks a lot for Martin Potter to find and report the race condition
which was causing the problem.

ServicePointManager.RecycleServicePoints() was a left-over from the
old times when we didn't have an idle-timer based approach.