[LLDP]: Modify OID index of LLDPRemTableUpdater MIB (sonic-net#155)

* [LLDP]: LLDPRemTableUpdater index in OID is a tuple of
(if index, remote time mark, remote device index).
In case of multi-asic platform, when querying this MIB it can happen
that same if index result is seen in SNMP walk, with a different
remote time mark. To avoid showing same if index result in the
result, set remote time mark to 0.
* Add code comment.

src/sonic_ax_impl/mibs/ieee802_1ab.py

@@ -430,7 +430,12 @@ def update_data(self):
             if not lldp_kvs:
                 continue
             try:
-                time_mark = int(lldp_kvs[b'lldp_rem_time_mark'])
+                # OID index for this MIB consists of remote time mark, if_oid, remote_index.
+                # For multi-asic platform, it can happen that same interface index result 
+                # is seen in SNMP walk, with a different remote time mark.
+                # To avoid repeating the data of same interface index with different remote 
+                # time mark, remote time mark is made as 0 in the OID indexing.
+                time_mark = 0
                 remote_index = int(lldp_kvs[b'lldp_rem_index'])
                 self.if_range.append((time_mark,
                                       if_oid,