osd: add local_mtime to struct object_info_t

This fixes a bug when the time of the OSDs and clients are not
synchronized (especially when client is ahead of OSD), and the cache
tier dirty ratio reaches the threshold, the agent skips the flush work
because it thinks the object is too young.

Signed-off-by: Zhiqiang Wang <wonzhq@hotmail.com>
Merge pull request #2188 from wonzhq/obj-mtime

osd: add local_mtime to struct object_info_t

Reviewed-by: Sage Weil <sage@redhat.com>