[release-branch.go1.8] net/http: update bundled http2 for gracefulShutdownCh lock contention slowdown

This updates the bundled x/net/http2 repo to git rev 186fd3fc (from
the net repo's release-branch.go1.8) for:

    [release-branch.go1.8] http2: fix lock contention slowdown due to gracefulShutdownCh
    https://golang.org/cl/43459

Fixes #20302

Change-Id: Ia01a44c6749292de9c16ca330bdebe1e52458b18
Reviewed-on: https://go-review.googlesource.com/43996
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>