rfc3: loopback routing clarification, draft status

Change RFC status from raw -> draft.

Clean up Goals section somewhat.  Specify an actual latency target.
Say "minimizing heap allocations" instead of "malloc free".

Clarify that FLUX_NODEID_ANY requests are never routed back to the
sender; they go up the TBON.

Finally, add some text before the ABNF that describes each message
part.  Figuring it all out from the ABNF might be a little much to ask
of a casual reader.