raft: Make PartitionKey opaque
Makes the composite PartitionKey into a single opaque value. The storage_name and partition_id in the original PartitionKey only made sense in the context of the storage which created it; once the partition is replicated, it can undergo leadership changes which render the values obsolete.
Instead, we can hash the storage_name and partition_id at the time of creation and pass around the single value as the key.
Edited by James Liu