Redis / PostgresQL reward caching; performance comparison

[pstjohn]

Would be great to get a quantitative estimation of the performance for redis / postgresQL, perhaps some wrapper functions that make caching easier using either backend.

I've noticed that sometimes the reward calculation takes a while -- with many simultaneous games, we sometimes see the entry not present in the reward buffer, but it gets added by a different node by the time its complete. so this can throw errors

Other things we could cache depending on performance:

• node values and prior logits (these would have to get cleared every time the model weights are updated)