ripd: implement northbound callbacks to fetch neighbor information

ripd: implement northbound callbacks to fetch neighbor information

Support for fetching operational data is experimental at this point.
Locks must be introduced to ensure the peer_list global variable 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>