Turns out that `pool.Put(buf)` had to *allocate* because we needed to turn
`[]byte` into `interface{}`. Apparently, we've never done this correctly we just
never noticed because we never really used buffer pools extensively.

However, since migrating yamux to a buffer-pool backed buffer, this started
showing up in allocation profiles.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>