restart or stopping the snmp-agent generates call traceback with syslog ERROR

[bsbsrkg]

Description

Steps to reproduce the issue:

1. docker exec snmp supervisorctl stop snmp-agent or docker exec snmp supervisorctl restartsnmp-agent

Describe the results you received:
Got python traceback error with syslog ERR.

Dec 18 12:54:16.079672 str-msn2700-01 INFO snmp#snmp-subagent [sonic_ax_impl] INFO: Recieved 'SIGTERM' signal, shutting down...
Dec 18 12:54:16.080600 str-msn2700-01 INFO snmp#snmp-subagent [ax_interface] INFO: AgentX socket connection closed.
Dec 18 12:54:16.081329 str-msn2700-01 INFO snmp#snmp-subagent [ax_interface] INFO: Run disabled. Connection loop stopping...
Dec 18 12:54:16.087179 str-msn2700-01 ERR snmp#snmp-subagent [sonic_ax_impl] ERROR: Uncaught exception in sonic_ax_impl.main
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/main.py", line 70, in main
    event_loop.run_until_complete(agent.run_in_event_loop())
  File "/usr/lib/python3.6/asyncio/base_events.py", line 466, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.6/dist-packages/ax_interface/agent.py", line 49, in run_in_event_loop
    await asyncio.wait_for(background_task, BACKGROUND_WAIT_TIMEOUT, loop=self.loop)
  File "/usr/lib/python3.6/asyncio/tasks.py", line 352, in wait_for
    return fut.result()
AttributeError: '_asyncio.Task' object has no attribute 'send'
Dec 18 12:54:20.016122 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:16,078 INFO waiting for snmp-subagent to stop
Dec 18 12:54:20.016548 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:18,078 INFO waiting for snmp-subagent to stop
Dec 18 12:54:30.032563 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:20,079 INFO waiting for snmp-subagent to stop
Dec 18 12:54:30.032563 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:22,079 INFO waiting for snmp-subagent to stop
Dec 18 12:54:30.032563 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:24,079 INFO waiting for snmp-subagent to stop
Dec 18 12:54:30.032563 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:26,078 WARN killing 'snmp-subagent' (76) with SIGKILL
Dec 18 12:54:30.032563 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:26,079 INFO waiting for snmp-subagent to stop
Dec 18 12:54:30.032563 str-msn2700-01 INFO snmp#supervisord 2016-12-18 12:54:26,083 INFO stopped: snmp-subagent (terminated by SIGKILL)

Describe the results you expected:
No traceback message and syslog ERR logs expected

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

SONiC Software Version: SONiC.HEAD.7-c9275173
Distribution: Debian 9.9
Kernel: 4.9.0-8-2-amd64
Build commit: c9275173
Build date: Sat Jun 15 09:57:18 UTC 2019
Built by: johnar@jenkins-worker-4

Platform: x86_64-accton_as7712_32x-r0
HwSKU: Accton-AS7712-32X
ASIC: broadcom
Serial Number: 771232X1633006
Uptime: 12:21:07 up  3:18,  3 users,  load average: 0.35, 0.33, 0.39

Docker images:
REPOSITORY                 TAG                 IMAGE ID            SIZE
docker-syncd-brcm          HEAD.7-c9275173     dcd654ca4343        392MB
docker-syncd-brcm          latest              dcd654ca4343        392MB
docker-lldp-sv2            HEAD.7-c9275173     e1b3912e86f8        299MB
docker-lldp-sv2            latest              e1b3912e86f8        299MB
docker-dhcp-relay          HEAD.7-c9275173     49d9c70412da        288MB
docker-dhcp-relay          latest              49d9c70412da        288MB
docker-database            HEAD.7-c9275173     d764ed141abf        280MB
docker-database            latest              d764ed141abf        280MB
docker-snmp-sv2            HEAD.7-c9275173     137d0904759d        313MB
docker-snmp-sv2            latest              137d0904759d        313MB
docker-orchagent           HEAD.7-c9275173     61ff16fd4581        319MB
docker-orchagent           latest              61ff16fd4581        319MB
docker-teamd               HEAD.7-c9275173     a3b32b85f53a        301MB
docker-teamd               latest              a3b32b85f53a        301MB
docker-sonic-telemetry     HEAD.7-c9275173     69fc95f17751        302MB
docker-sonic-telemetry     latest              69fc95f17751        302MB
docker-router-advertiser   HEAD.7-c9275173     3fb74e2b862b        280MB
docker-router-advertiser   latest              3fb74e2b862b        280MB
docker-platform-monitor    HEAD.7-c9275173     1b6410dd4202        326MB
docker-platform-monitor    latest              1b6410dd4202        326MB
docker-fpm-frr             HEAD.7-c9275173     4187e87f0e00        309MB
docker-fpm-frr             latest              4187e87f0e00        309MB
docker-ptf                 latest              00938f526d99        628MB

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```