net/http: fix wantConnQueue memory leaks in Transport
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.