Although an AEAD, in general, can be used concurrently in both the seal
and open directions, TLS is easier. Since the transport keys are
different for different directions in TLS, an AEAD will only ever be
used in one direction. Thus we don't need separate buffers for seal and
open because they can never happen concurrently.

Also, fix the nonce size to twelve bytes since the fixed-prefix
construction for AEADs is superseded and will never be used for anything
else now.

Change-Id: Ibbf6c6b1da0e639f4ee0e3604410945dc7dcbb46
Reviewed-on: https://go-review.googlesource.com/30959
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>