…f it is not supported by a pool

Currently, SAI_BUFFER_POOL_STAT_WATERMARK_BYTES and SAI_BUFFER_POOL_STAT_XOFF_ROOM_WATERMARK_BYTES are queried for buffer pools.
However, the latter is not supported on all pools and all platforms, which will results in sairedis complaint
To avoid that, we need to test whether it is supported before putting it to FLEX_COUNTER table

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

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

The latter is not supported by all vendors

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

…rk polling

- If shared headroom pool watermark is not supported,
  only buffer pool watermark will be tested
- Otherwise, both buffer pool watermark and shared headroom pool watermark will be checked

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

Fix review comments.

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