lib/snmp: (no technical change) renaming functions

rename:
fill -> get
fill_tics -> get_tics
fill_date -> get_date
fill_asn -> get_value
fill_struct_pp_asn -> get_pp
fill_struct_p_asn -> get_p
fill_int32_saturate -> get_i32sat
fill_int32_saturate_pp -> get_i32sat_pp

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>