ripd: implement northbound callbacks to fetch route information

ripd: implement northbound callbacks to fetch route information

Support for fetching operational data is experimental at this point.
Locks must be introduced to ensure the rip->table routing table won't
be modified while we're iterating asynchronously over it (or iterating
from a separate pthread).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>