Reuse buffer in benchmarks.

This seems to give us a better view of what's actually happening in this
test.

Before:

ReceiveRequest         387 ns/op  105.68 MB/s  115 B/op  3 allocs/op
ReceiveResponse        294 ns/op  139.13 MB/s   82 B/op  2 allocs/op
ReceiveResponseNoBuf   396 ns/op  103.41 MB/s  115 B/op  3 allocs/op

After:

ReceiveRequest         299 ns/op  136.81 MB/s   64 B/op  2 allocs/op
ReceiveResponse        204 ns/op  200.57 MB/s   32 B/op  1 allocs/op
ReceiveResponseNoBuf   296 ns/op  138.40 MB/s   64 B/op  2 allocs/op