osd: tiering: do not block proxy read on background promotion

Allow proxy read when there is no op blocked on the promotion
(background promotion). Also, when the promotion is done, kick off the
blocked proxy reads to preserve op ordering.

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
osd: tiering: cancel and requeue proxy read when needed

Cancel and requeue proxy read on the following cases:
1) on_shutdown
2) on_change
3) background promotion is done

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
osd: tiering: proxy instead of redirect read in writeback mode when the
cache pool is full

To preserve read op order

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
osd: tiering: avoid duplicate promotion on proxy read

Do not promote if it is already undergoing in maybe_handle_cache.

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>