Add a new peerMaxHeaderListSize member to ClientConn which records the
SETTINGS_MAX_HEADER_LIST_SIZE requested by the client's peer, and
respect this limit in (*ClientConn) encodeHeaders / encodeTrailers.

Attempting to send more than peerMaxHeaderListSize bytes of headers or
trailers will result in RoundTrip returning errRequestHeaderListSize.

Updates golang/go#13959

Change-Id: Ic707179782acdf8ae543429ea1af7f4f30e67e59
Reviewed-on: https://go-review.googlesource.com/29243
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>