[snmp/agent] Renamed the mib_data (backend) modules

Renmed the two backend modules:

       snmpa_mib_data_tree1.erl -> snmpa_mib_data_tttn.erl
       snmpa_mib_data_tree2.erl -> snmpa_mib_data_ttln.erl

TTTN - TupleTreeTupleNodes
TTLN - TupleTreeListNodes
[snmp/agent] Preliminary doc update regarding mib_data behaviour
[snmp/agent] Some minor mib_data behaviour changes