I just saw this message "transaction rescued from peer latency cache". Can someone explain what it means? Is it some new function in v8?
Yes, that message is a result of a new feature in v8:
* Transaction preforwarding Transactions that you're mining are sent to peers before you get a share, so any block solution you find can be broadcast virtually instantaneously. This could theoretically get our invalid block rate below any other pool's thanks to our large network of nodes.
P2Pool nodes indicate to their peers which transactions they have in memory so that peers don't need to send the full transaction when, for example, they produce a share containing that transaction. This is to reduce latency below what would result from using something like Bitcoin's "inv"/"getdata" messages, where sending a peer a transaction it doesn't yet know about takes three round-trips instead of one.
"Transaction rescued from peer latency cache" means that your node had a transaction in its memory, determined that it was no longer necessary to retain it, and forgot about it. Meanwhile, a peer is trying to forward the transaction to you and sees that you already have it, so it sends only the transaction hash instead of the full transaction. Were it not for the first node buffering the transaction for a few seconds before truly forgetting about it, sending the transaction hash instead of the full transaction would result in the first node not knowing about the transaction.