osd: basic cache tier agent

This branch includes a basic cache tier agent.  It isn't doing anything
smart about object temperature (how frequently it is accessed) but it is
reasonably smart about how recently objects were accessed, and it puts the
basic infrastruture in place.

Missing components:

 - better prioritization of the agent work thread.  not sure what the best
   approach here is.
 - some temperature smarts. i think we need to define another policy paramter
   to control how the agent weighs atime vs temp.. and i think we also need to
   add a creation timestamp in object_info_t to make that work.
 - only a single thread initiating agent work.