net-snmp2 util implemented

 - get, get_next, get_bulk, walk, set supported
 - Multiple sessions supported via `manage` & `close`
 - MIB operations supported with `describe` & `translate`

All operations seem to agree with output from snmp[operation], though not
alwasy with MIBBrowser. (MIBBrowser seems to accept responses from walks
that are not part of the subtree requested for walking.)

Some trap unit tests are failing. Need to verify behavior on master, and
possibly fix. Though, no trap logic should have been affected.