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>
Merge pull request #2968 from wonzhq/proxy-read-op-order

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