[pfcwd] Refactor pfcwd

[wendani]

What I did

1. Wrap config processing into the following 4 newly introduced member functions with NO change or modification to the current processing logic

• registerPortInWdDb
• registerQueueInWdDb
• unregisterPortFromWdDb
• unregisterQueueFromWdDb

2. Change data member m_entryMap and m_brsEntryMap from map to unordered_map

Why I did it Pave the way for https://github.com/Azure/sonic-swss/pull/1697

How I verified it On brcm

Details if related This PR introduces NO change or modification to the current pfcwd orch processing logic.

Contains and therefore after https://github.com/Azure/sonic-swss/pull/1691

• [ ] https://github.com/Azure/sonic-swss/pull/1691

[lgtm-com[bot]]

This pull request fixes 2 alerts when merging c68db83f998273c337a3613ed6addc9eb4dcead7 into 66e1aab8689de4ee532a8301b8bb8414519fdf24 - view on LGTM.com

fixed alerts:

• 2 for Unused import