[Observium] Cisco-vrf discovery - huge performance improvement

Hi,

As I wanted to make Cisco VRF discovery SNMPv3-compatible, I realized that it was actually very slow. So I change the philosophy of it (bulkwalk all SNMP info first then analyze in the code), and here's the result for a device with 500 VRFs:

Old version : Discovered in 65.83 seconds New version : Discovered in 3.234 seconds

At the same time I made it SNMPv3-compatible, and I added the support for CISCO-L3VPN-STD-MIB that is the more recent MIB and the only one supported on the latest releases of IOS/IOS-XR.

I tested it with Cisco Hw and it's fine, but for the other vendors implementing the MIB, somebody else will have to test it :)

Guillaume