[snmp/agent] New behaviour module (snmpa_mib_data) and two impl mods

The old mib-server data module (snmpa_mib_data) has been changed into
a behaviour module. The old implementation is now instead in a new
module snmpa_mib_data_tree1. A new backend module has been added,
still only very preliminary, snmpa_mib_data_tree2.