Don't filter out nodes by score only once in the balanced node selector

[sfackler]

To make the implementation more simple, the balanced node selector currently only filters out nodes with high scores once when first building its future, and then selects only on the non-filtered nodes from there. This makes the selector slightly less responsive to score changes while a request is processing which is in theory bad. It's not clear how much this actually matters though.

Fixing it would probably involve wrapping the node score in an async semaphore, and having the future have state for each node that's always waiting for the score to change, and then also waiting on the limiter for nodes that score sufficiently well.