PFC-WD class cleanup

[rbpittman]

What I did

• Merge the duplicate PfcWdDlrHandler and PfcWdSaiDlrInitHandler handlers.
• Cisco will no longer use the zero buffer profile handler inheritance chain as a result, primarily using the dlr attribute instead.
• The getHwCounters is promoted to the base class so the inheritance chain can be simpler and edge cases can be removed in the lossy handling.

Why I did it
To remove the unneeded zero buffer pool/profile application on cisco devices.

How I verified it
Informally verified counters are working and watchdog works on Cisco-8122 so far.
TODO: Performing more verification via tests on other asics.
TODO for msft: Broadcom validation when its dlr mode is enabled?

Details if related