[counters] interface and pfc counters are shown negative after config…

[AmitKaushik7]

… reload

- What I did
During syncd docker init, initialize the interface and pfc counters to 0.
- How I did it
By clearing the counters values in /tmp/portstat*/* amd /tmp/pfcstat*/* files.
- How to verify it
For Interface counters-
Send traffic for a couple of minutes on few ports
Check the counters values using show interface counters,
Clear the counters "sow interface counters -c"
Perform config reload
Check the counters values using show interface counters, values are negative.

For PFC counters-
Create congestion on a port by sending traffic from 2 ports to 1 port on no-drop priorities
Check the counters values using show pfc counters
Clear the counters using "show pfc counters -c"
Perform config reload
Check the counters values using show pfc counters, values are negative.

- Description for the changelog
Initializes ps and pfcstat counters during docker initialization.

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