This drops the number of select cases in serverConn.server from 10 to
6. It was 7 in Go 1.7. It increased to 10 in Go 1.8.

* replace testing-only testHookCh with always-on grab-bag serveMsgCh
  to be used for both test purposes, and infrequently used message
  types

* remove the settingsTimer.C case for the initial settings timer
  and use serveMsgCh and time.AfterFunc instead

* ... and do the same for the idle timeout timer.

* ... and for the shutdown timer.

* remove wantStartPushCh and just send the *startPushRequest to
  serveMsgCh too

I could go further with this (and plan to, later), but these are the
safe and easy ones that don't require more work elsewhere.

The speed gets better the more the request/response go via the
serverConn.serve loop. (once per Request.Body.Read or
ResponseWriter.Flush):

  name            old time/op    new time/op  delta
  ServerGets-4    138µs ± 3%     134µs ± 2%   -2.54%   (p=0.002 n=10+10)
  ServerPosts-4   176µs ±27%     154µs ± 2%   -12.62%  (p=0.011 n=10+10)

Updates kubernetes/kubernetes#45216
Updates golang/go#20302

Change-Id: I18019554089d7e3d76355d7137b5957e9597e803
Reviewed-on: https://go-review.googlesource.com/43034
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>