Improve the way of handling BUFFER_PG during PFC storm

[stephenxs]

What I did
One action for the PFC storm is to set a zero buffer profile on the PG under PFC storm.
The zero buffer won't be removed until the PFC storm has gone.
If the user wants to modify the buffer profile for the PG, the bufferorch will return "task_need_retry".
General speaking it doesn't matter unless that:

• the system can't be warm rebooted until the PFC storm has gone.
• the "task_need_retry" will block the update coming from any entry of the BUFFER_PG table from being programmed to ASIC.
  In this sense, we need a better solution.

In the new solution, it will:

• record the new profile user wants to apply during PFC storm as the "pending profile" for that PG and return "task_success" if the PG is under PFC storm.
• apply the pending profile once the PG is unlocked.
• the latest pending profile will take effect in case user tries updating the profile for more than 1 times.

Signed-off-by: Stephen Sun stephens@nvidia.com

Why I did it

How I verified it
It can be verified in the following steps:

1. configure PFC watchdog. the buffer profile should be the original value
2. start PFC storm on PG 4.

NOTICE swss#orchagent: :- startWdActionOnQueue: Receive notification, storm
NOTICE swss#orchagent: :- startWdActionOnQueue: PFC Watchdog detected PFC storm on port Ethernet64, queue index 4, queue id 0x150000000006e5 and port id 0x10000000006de.

3. modify PG 3-4

Oct 16 10:31:50.869634 mtbc-sonic-01-2410 NOTICE swss#orchagent: :- processPriorityGroup: Set buffer PG Ethernet64:3-4 to profile
Oct 16 10:31:50.869893 mtbc-sonic-01-2410 WARNING swss#orchagent: :- processPriorityGroup: Priority group 4 on port Ethernet64 is locked, pending until unlocked

4. stop PFC storm

Oct 16 10:33:29.099110 mtbc-sonic-01-2410 NOTICE swss#orchagent: :- startWdActionOnQueue: Receive notification, restore
Oct 16 10:33:29.099110 mtbc-sonic-01-2410 NOTICE swss#orchagent: :- startWdActionOnQueue: PFC Watchdog storm restored on port Ethernet64, queue index 4, queue id 0x150000000006e5 and port id 0x10000000006de.
Oct 16 10:33:29.117272 mtbc-sonic-01-2410 NOTICE swss#orchagent: :- ~PfcWdZeroBufferHandler: Priority group 4 on port Ethernet64 has been resotred to pending profile 0x19000000000ab1

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