Fix exception when running snmp subagent under python3.7 (#130)

May  5 21:35:16.852142 sonic ERR snmp#snmp-subagent [sonic_ax_impl] ERROR: Uncaught exception in sonic_ax_impl.main#012Traceback (most recent call last):#12  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/main.py", line 70, in main#012    event_loop.run_until_complete(agent.run_in_event_loop())#12  File "/usr/lib/python3.7/asyncio/base_events.py", line 584, in run_until_complete#012    return future.result()#12  File "/usr/local/lib/python3.7/dist-packages/ax_interface/agent.py", line 37, in run_in_event_loop#012    background_task = self.mib_table.start_background_tasks(self.oid_updaters_enabled)#12  File "/usr/local/lib/python3.7/dist-packages/ax_interface/mib.py", line 276, in start_background_tasks#012    task = event._loop.create_task(fut)#12  File "/usr/lib/python3.7/asyncio/base_events.py", line 405, in create_task#012    task = tasks.Task(coro, loop=self)#012TypeError: a coroutine was expected, got <Task pending coro=<MIBUpdater.start() running at /usr/local/lib/python3.7/dist-packages/ax_interface/mib.py:34> cb=[MIBTable._done_background_task_callback() at /usr/local/lib/python3.7/dist-packages/ax_interface/mib.py:263]>

We really do not need to wrap the future within a task.

This also addresses the issue where the subagent would not exit on
SIGTERM.

src/ax_interface/mib.py

@@ -281,8 +281,7 @@ def start_background_tasks(self, event):
             updater.run_event = event
             fut = asyncio.ensure_future(updater.start())
             fut.add_done_callback(MIBTable._done_background_task_callback)
-            task = event._loop.create_task(fut)
-            tasks.append(task)
+            tasks.append(fut)
         return asyncio.gather(*tasks, loop=event._loop)
 
     def _find_parent_prefix(self, item):