Reuse artifact buffers

[derekchiang]

In networked systems that need access to fixed-size buffers, it's common to use a buffer pool to minimize allocation and relieve GC pressure. Go has sync.Pool that's good for this purpose. It could be an over-optimization though since it's unclear if any peer would ever allocate buffers fast enough that memory actually becomes an issue.

[derekchiang]

I was looking at this code for instance.