[pfcwd] Observe pfc enable bits change

[wendani]

What I did
Implement 2.6.3 Queue Reconfiguration or Removal in PFC watchdog HLD

Why I did it
Fix issue 2 stated in #842: "A mechanism should be introduced to let PFCWD respond to this event"

How I verified it
vs test and on brcm

vs test the following scenarios:

1. Set PFC WD config on port before set any PFC enable bit
2. Set PFC enable on {port, TC 3}
3. Clear PFC enable on {port, TC 3}, and set PFC enable on {port, TC 4}
4. Set big red switch mode on top of PFC WD config on port
5. Clear PFC enable on {port, TC 4}, and set PFC enable on {port, TC 3}
6. Clear all PFC enable bits (i.e., Clear PFC enable on {port, TC 3})

At each PFC enable bit change step, check

• port and queue level entry in FLEX_COUNTER_DB
• COUNTERS_DB PFC_WD_STATUS, PFC_WD_QUEUE_STATS_DEADLOCK_DETECTED, PFC_WD_QUEUE_STATS_DEADLOCK_RESTORED value
• APPL_DB queue in-storm status

Details if related
Let pfcwd orch be an observer of qos orch PFC enable bits change, and update db registration accordingly.

Fix #842

Peel off current code refactoring part in PR #1696

• [pfcwd] Refactor pfcwd #1696

[lgtm-com]

This pull request fixes 2 alerts when merging 4ce1bce into 66e1aab - view on LGTM.com

fixed alerts:

• 2 for Unused import

[lguohan]

@wendani , can you do a proper pr, too many commits in your pr. https://www.thinkful.com/learn/github-pull-request-tutorial/#:~:text=Time%20to%20Submit%20Your%20First%20PR%201%20Go,github%20username%3E%2Fguides-github-pull-request.git%20in%20your%20terminal%20More%20items...%20