I've put together a high level planning doc for 5.1. Please have a look and see if I've missed out on anything. Any other high-level JIRAs should be linked to this doc, and if JIRAs for certain things don't exist, please create them.
"and adding a digesting mechanism so that unchanged entries that can be retrieved locally, such as from a cache store, need not be transferred."
AFAIK this can only be done for replicated caches, or distributed caches that have a shared cache store, so my understanding was that the priority of this was downgraded.