Make snmpit_cisco much, much more robust to failures.
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.
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