crypto/tls: Use the same buffer size in the client and server in the TLS throughput benchmark

I believe it's necessary to use a buffer size smaller than 64KB because
(at least some versions of) Window using a TCP receive window less than
64KB. Currently the client and server use buffer sizes of 16KB and 32KB,
respectively (the server uses io.Copy, which defaults to 32KB internally).
Since the server has been using 32KB, it should be safe for the client to
do so as well.

Fixes #15899

Change-Id: I36d44b29f2a5022c03fc086213d3c1adf153e983
Reviewed-on: https://go-review.googlesource.com/24581


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>