[RFE] Rework SNMP agent to base on version-agnostic SNMP API

The VirtualPDU tool implements SNMP agent to simulate SNMP-enabled power distribution units. Current implementation is based on the low-level API of the pysnmp library. That limits SNMP agent capabilities in many ways, most importantly SNMPv3 and its security features can't be supported.

This bug is to rework current implementation to make it relying upon high-level pysnmp API. Technically, this probably involves using SNMP CommandResponder application along with SNMP engine entity.