Get the currently atime of the object in cache pool for eviction

Because if there are mutiple atime in agent_state for the same object, we should use the recently one.

Signed-off-by: Xinze Chi <xmdxcxz@gmail.com>
Merge pull request #2816 from XinzeChi/master

Get the currently atime of the object in cache pool for eviction 

Backport: giant, firefly
Reviewed-by: Sage Weil <sage@redhat.com>