Convert most SNMP interaction to use the snmpit*() library, so that
they get support for retrying failures, etc.

Add new library calls for wrapping bulkwalk() - so now, we will
retry those on error as well. Before, we had the bad behavior than
many functions, like listVlans() would just see empty lists instead
of errors.

When making a new Cisco object, we now test network connectivity right
away, by fetching an OID that should exist on all SNMP devices. Before,
we wouldn't find out we couldn't contact the switch until we actually
did something on it.

Also, make VLAN number choosing go a bit faster by converting it to
bulkwalk() (using the new library function) so we can grab all VLAN
numbers at once.