Node-level shard lookup for persist requests #6118
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Once requests are routed to nodes, we can have the node try to find the shard with the most capacity, to better load balance requests. Since the node is the source of truth for tracking its own shards' capacity, this should be a minor to moderate optimization to better pack persist requests between nodes.
This requires removing the "Tracked" state on the publish tracker and moving that logic into the persisted state. It should be a no-op, but introduces the (seemingly impossible) chance that if a publish arrives before a persist, we'd drop it.
The other "hack" here is that the persist subrequests still target an individual shard, but that shard isn't necessarily the one we end up persisting to. This isn't a problem in the literal sense because PersistSuccesses do report the correct shard, and the publish tracker keeps track per subrequest and then updates from the shard within the subrequest. But there will be some logical misdirection there.
Updated tests to pass. Next step is testing in staging.