[202012][dockers][supervisor] Increase event buffer size for process exit listener; Set all event buffer sizes to 1024

[jleveque]

Why I did it

Backport of #7083 to the 202012 branch.

To prevent error messages like the following from being logged:

Mar 17 02:33:48.523153 vlab-01 INFO swss#supervisord 2021-03-17 02:33:48,518 ERRO pool supervisor-proc-exit-listener event buffer overflowed, discarding event 46

This is basically an addendum to #5247, which increased the event buffer size for dependent-startup. While supervisor-proc-exit-listener doesn't subscribe to as many events as dependent-startup, there is still a chance some containers (like swss, as in the example above) have enough processes running to cause an overflow of the default buffer size of 10.

This is especially important for preventing erroneous log_analyzer failures in the sonic-mgmt repo regression tests, which have started occasionally causing PR check builds to fail. Example here.

I set all supervisor-proc-exit-listener event buffer sizes to 1024, and also updated all dependent-startup event buffer sizes to 1024, as well, to keep things simple, unified, and allow headroom so that we will not need to adjust these values frequently, if at all.

[jleveque]

/azp run

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