1. Connections and codecs need to be partially safe for concurrent use.
   Namely, read side is serialized by one mutex,
   and writing side is serialized by another.
   Current comment says that they need to be fully thread-safe,
   which makes the default implementations (gobClientCodec/gobServerCodec)
   non-conforming.

2. Say that ServerCodec.Close can be called multiple times
   and must be idempotent. Server requires this and gobServerCodec
   accounts for this,  but the requirement is not documented.

Change-Id: Ie877e37891fed28056e3d9d1722edaed8e154067
Reviewed-on: https://go-review.googlesource.com/120818Reviewed-by: Rob Pike <r@golang.org>