com.twitter.finagle.partitioning
When given a partitionId, asynchronously returns a Service that sends requests to a shard within that partition.
When given a partitionId, asynchronously returns a Service that sends requests to a shard within that partition.
we assume that the factory has implemented FactoryToService under the covers, so the returned service does not need to be closed by the caller.
Represents the snapshot of the information needed to determine how to carve up the request, and where to send those carved-up slices to.