When stopping the swss, pmon or bgp containers, log messages like the following can be seen:
```
Aug 23 22:50:43.789760 sonic-dut INFO swss#supervisord 2020-08-23 22:50:10,061 ERRO pool dependent-startup event buffer overflowed, discarding event 34
Aug 23 22:50:43.789760 sonic-dut INFO swss#supervisord 2020-08-23 22:50:10,063 ERRO pool dependent-startup event buffer overflowed, discarding event 35
Aug 23 22:50:43.789760 sonic-dut INFO swss#supervisord 2020-08-23 22:50:10,064 ERRO pool dependent-startup event buffer overflowed, discarding event 36
Aug 23 22:50:43.789760 sonic-dut INFO swss#supervisord 2020-08-23 22:50:10,066 ERRO pool dependent-startup event buffer overflowed, discarding event 37
```
This is due to the number of programs in the container managed by supervisor, all generating events at the same time. The default event queue buffer size in supervisor is 10. This patch increases that value in all containers in order to eliminate these errors. As more programs are added to the containers, we may need to further adjust these values. I increased all buffer sizes to 25 except for containers with more programs or templated supervisor.conf files which allow for a variable number of programs. In these cases I increased the buffer size to 50. One final exception is the swss container, where the buffer fills up to ~50, so I increased this buffer to 100.
Resolves https://github.com/Azure/sonic-buildimage/issues/5241
**- Why I did it**
Initially, the critical_processes file contains either the name of critical process or the name of group.
For example, the critical_processes file in the dhcp_relay container contains a single group name
`isc-dhcp-relay`. When testing the autorestart feature of each container, we need get all the critical
processes and test whether a container can be restarted correctly if one of its critical processes is
killed. However, it will be difficult to differentiate whether the names in the critical_processes file are
the critical processes or group names. At the same time, changing the syntax in this file will separate the individual process from the groups and also makes it clear to the user.
Right now the critical_processes file contains two different kind of entries. One is "program:xxx" which indicates a critical process. Another is "group:xxx" which indicates a group of critical processes
managed by supervisord using the name "xxx". At the same time, I also updated the logic to
parse the file critical_processes in supervisor-proc-event-listener script.
**- How to verify it**
We can first enable the autorestart feature of a specified container for example `dhcp_relay` by running the comman `sudo config container feature autorestart dhcp_relay enabled` on DUT. Then we can select a critical process from the command `docker top dhcp_relay` and use the command `sudo kill -SIGKILL <pid>` to kill that critical process. Final step is to check whether the container is restarted correctly or not.
Introduce a new "sflow" container (if ENABLE_SFLOW is set). The new docker will include:
hsflowd : host-sflow based daemon is the sFlow agent
psample : Built from libpsample repository. Useful in debugging sampled packets/groups.
sflowtool : Locally dump sflow samples (e.g. with a in-unit collector)
In case of SONiC-VS, enable psample & act_sample kernel modules.
VS' syncd needs iproute2=4.20.0-2~bpo9+1 & libcap2-bin=1:2.25-1 to support tc-sample
tc-syncd is provided as a convenience tool for debugging (e.g. tc-syncd filter show ...)
