Fix a regression from Go 1.5 to Go 1.6: when we introduced automatic
HTTP/2 support in Go 1.6, we never handled Server.ReadTimeout. If a
user set ReadTimeout, the net/http package would set the read deadline
on the connection during the TLS handshake, but then the http2 package
would never disarm it, killing the likely-still-in-use connection
after the timeout period.

This CL changes it to disarm the timeout after the first request
headers, similar to net/http.Server.

Unlike net/http.Server, we don't re-arm it on each idle period,
because the definition of idle is more complicated with HTTP/2.

No tests here for now. Tests will be in the go repo once all the knobs
are in place and this is re-bundled into std, testing both http1 and
http2.

Updates golang/go#16450 (minimal fix for now)
Updates golang/go#14204 (considering a new http1+http2 IdleTimeout knob)

Change-Id: Iaa1570c118efda7dc0a65ba84cd77885699ec8fc
Reviewed-on: https://go-review.googlesource.com/30077Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>