osd/ReplicatedPG: basic flush and evict agent functionality

osd/ReplicatedPG: basic flush and evict agent functionality

This is very basic flush and evict functionality for the tiering agent.

The flush policy is very simple: if we are above the threshold and the
object is dirty, and not super young, flush it.  This is not too braindead
of a policy (although we could clearly do something smarter).

The evict policy is pretty simple: evict the object if it is clean and
we are over our full threshold.  If we are in the middle mode, try to
estimate how cold the object is based on an accumulated histogram of
objects we have examined so far, and decide to evict based on our
position in that histogram relative to our "effort" level.

Caveats:
 * the histograms are not refreshed
 * we aren't taking temperature into consideration yet, although some of
   the infrastructure is there.

Signed-off-by: Sage Weil <sage@inktank.com>