#### Why I did it
When CPU is busy, the sonic_ax_impl may not have sufficient speed to handle the notification message sent from REDIS.
Thus, the message will keep stacking in the memory space of sonic_ax_impl.
If the condition continues, the memory usage will keep increasing.
#### How I did it
Add a monit file to check if the SNMP container where sonic_ax_impl resides in use more than 4GB memory.
If yes, restart the sonic_ax_impl process.
#### How to verify it
Run a lot of this command: `while true; do ret=$(redis-cli -n 0 set LLDP_ENTRY_TABLE:test1 test1); sleep 0.1; done;`
And check the memory used by sonic_ax_impl keeps increasing.
After a period, make sure the sonic_ax_impl is restarted when the memory usage reaches the 4GB threshold.
And verify the memory usage of sonic_ax_impl drops down from 4GB.
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