rpclib: Use prop351 protocol to open streams.

Now that prop351 is what Arti speaks, it's what the rpclib needs to provide.

Note one change in particular: the isolation string is no longer an optional argument when opening a stream. (With prop351, there is no longer such a thing as an "absent" isolation string, and we don't want to imply that there is a difference between None and "".)

This is a pretty short commit and should be straightforward.

Followup from !2401.