Replace packet UUID with monotonic packet sequence number about prio-server

[tgeoghegan]

@yuriks -- thoughts on this? This is the code I was writing that led me to this idea.

[tgeoghegan]

Having thought about this more, I think the right thing to do here is to expect that the order of UUIDs as they are in the ingestion batch will be maintained. It increasingly seems as though the prio-server implementation will be used for both PHA server and facilitator, so we can reasonably provide that guarantee, and relying on it allows individual servers to avoid the O(n log n) sort I described earlier. So let's keep the UUID. Closing this.