I'm trying to keep the code changes minimal for backporting to Go 1.13,
so it is still possible for a handful of entries to leak,
but the leaks are now O(1) instead of O(N) in the steady state.

Longer-term, I think it would be a good idea to coalesce idleMu with
connsPerHostMu and clear entries out of both queues as soon as their
goroutines are done waiting.

Fixes #33849
Fixes #33850

Change-Id: Ia66bc64671eb1014369f2d3a01debfc023b44281
Reviewed-on: https://go-review.googlesource.com/c/go/+/191964
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>