Fix sc.state, which was returning "idle" instead of "closed" if the max
pushed stream ID happened to exceed the max client stream ID. This caused
us to erroneously generate connection errors because we believed a state
invariant had been violated when it had not.

I also renamed maxStreamID to maxClientStreamID for clarity, since we
need to track client-generated and server-generated stream IDs separately.

Fixes golang/go#17777

Change-Id: Id3d5700d79cc699a267bd91d6ebace0770fa62fc
Reviewed-on: https://go-review.googlesource.com/32755Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>