crypto/tls: reduce size of buffer in throughput benchmarks
The Windows builders run the throughput benchmarks really slowly with a
64kb buffer. Lowering it to 16kb brings the performance back into line
with the other builders.
This is a work-around to get the build green until we can figure out why
the Windows builders are slow with the larger buffer size.
Update #15899
Change-Id: I215ebf115e8295295c87f3b3e22a4ef1f9e77f81
Reviewed-on: https://go-review.googlesource.com/23574 Reviewed-by: Ian Lance Taylor <iant@golang.org>