OPTIM: buffers: align the buffer pool to 64

OPTIM: buffers: align the buffer pool to 64

This struct is used by memcpy() and friends, particularly during the
early recv() and send(). By keeping it 64-byte aligned, we let the
underlying libs/kernel use optimal operations (e.g.  AVX512) for memory
copies while right now it's just random (buffers are found to be equally
aligned to 32 and 64 in practice).