Fix exception when running snmp subagent under python3.7

[joyas-joseph]

- What I did
As part of converting the snmp docker to buster, I observed that the snmp subagent exits after throwing an exception.

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):#012  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())#012  File "/usr/lib/python3.7/asyncio/base_events.py", line 584, in run_until_complete#012    return future.result()#012  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)#012  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)#012  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]>

The change also addresses the issue where snmp subagent doesnt exit on SIGTERM.

- How I did it
Code change.

- How to verify it
Verify that subagent is running in the docker.
snmpwalk -c public -v2c .1.3.6.1.2.1.2.2.1.2

- Description for the changelog
Fix exception when running snmp subagent under python3.7

[joyas-joseph]

This change has been verified in both pyhton3.6 and python3.7

$ snmpwalk -c public -v 2c 10.11.57.9 .1.3.6.1.2.1.2.2.1.2
iso.3.6.1.2.1.2.2.1.2.1 = STRING: "hundredGigE1/1"
iso.3.6.1.2.1.2.2.1.2.5 = STRING: "hundredGigE1/2"
iso.3.6.1.2.1.2.2.1.2.9 = STRING: "hundredGigE1/3"
iso.3.6.1.2.1.2.2.1.2.13 = STRING: "hundredGigE1/4"
iso.3.6.1.2.1.2.2.1.2.17 = STRING: "hundredGigE1/5"
iso.3.6.1.2.1.2.2.1.2.21 = STRING: "hundredGigE1/6"
iso.3.6.1.2.1.2.2.1.2.25 = STRING: "hundredGigE1/7"
iso.3.6.1.2.1.2.2.1.2.29 = STRING: "hundredGigE1/8"
iso.3.6.1.2.1.2.2.1.2.33 = STRING: "hundredGigE1/9"
iso.3.6.1.2.1.2.2.1.2.37 = STRING: "hundredGigE1/10"
iso.3.6.1.2.1.2.2.1.2.41 = STRING: "hundredGigE1/11"
iso.3.6.1.2.1.2.2.1.2.45 = STRING: "hundredGigE1/12"
iso.3.6.1.2.1.2.2.1.2.49 = STRING: "hundredGigE1/13"
iso.3.6.1.2.1.2.2.1.2.53 = STRING: "hundredGigE1/14"
iso.3.6.1.2.1.2.2.1.2.57 = STRING: "hundredGigE1/15"
iso.3.6.1.2.1.2.2.1.2.61 = STRING: "hundredGigE1/16"
iso.3.6.1.2.1.2.2.1.2.65 = STRING: "hundredGigE1/17"
iso.3.6.1.2.1.2.2.1.2.69 = STRING: "hundredGigE1/18"
iso.3.6.1.2.1.2.2.1.2.73 = STRING: "hundredGigE1/19"
iso.3.6.1.2.1.2.2.1.2.77 = STRING: "hundredGigE1/20"
iso.3.6.1.2.1.2.2.1.2.81 = STRING: "hundredGigE1/21"
iso.3.6.1.2.1.2.2.1.2.85 = STRING: "hundredGigE1/22"
iso.3.6.1.2.1.2.2.1.2.89 = STRING: "hundredGigE1/23"
iso.3.6.1.2.1.2.2.1.2.93 = STRING: "hundredGigE1/24"
iso.3.6.1.2.1.2.2.1.2.97 = STRING: "hundredGigE1/25"
iso.3.6.1.2.1.2.2.1.2.101 = STRING: "hundredGigE1/26"
iso.3.6.1.2.1.2.2.1.2.105 = STRING: "hundredGigE1/27"
iso.3.6.1.2.1.2.2.1.2.109 = STRING: "hundredGigE1/28"
iso.3.6.1.2.1.2.2.1.2.113 = STRING: "hundredGigE1/29"
iso.3.6.1.2.1.2.2.1.2.117 = STRING: "hundredGigE1/30"
iso.3.6.1.2.1.2.2.1.2.121 = STRING: "hundredGigE1/31"
iso.3.6.1.2.1.2.2.1.2.125 = STRING: "hundredGigE1/32"