stats: mark InPayload.Data and OutPayload.Data for deletion (experimental)

[dfawley]

cc @PapaCharlie

RELEASE NOTES:

• stats: deprecate InPayload.Data and OutPayload.Data; they were experimental and will be deleted in the next release

[PapaCharlie]

Thanks for this! LGTM. While you're in there, I think it should also probably be explicitly documented that the Payload field can also be modified after HandleRPC returns since it will be handed over to the user code. If the handler needs access to it after the fact, it needs to be copied.

[dfawley]

I think it should also probably be explicitly documented that the Payload field can also be modified after HandleRPC returns since it will be handed over to the user code. If the handler needs access to it after the fact, it needs to be copied.

Done!