Optimize number of requests to HyperSync

[DZakh]

1. Partitions for dynamic registrations don't merge well - the tooFarToMerge = 20_000 is too small for some cases, we should probably use prevQueryRange instead
2. //If there were no logs at all in the current page query then fetch the
   //timestamp of the heighest block accounted for
   Looks like we perform an unnecessary HyperSync query per logs requests
3. When performing a backfill for a multichain indexer, we should throttle quering of chains which reached the head