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This way, copying it won't copy the underlying pool.

Also, initialize it immediately instead of in an `init` function to ensure that
it's fully initialized immediately.

This has moved to libp2p

Per the documentation, this example *isn't* guaranteed to work (and, in
practice, it fails occasionally).

1. They're small.
2. We can avoid the lock (major speedup).
3. Better memory locality.
4. Much simpler.