osd/ReplicatedPG: block requests to cache PGs when they are full

osd/ReplicatedPG: block requests to cache PGs when they are full

If we are full and get a write request to a new object, put the op on a
wait list.  Wake up when the agent frees up some space.

Note that we do not block writes to existing objects.  That would be a
more aggressive strategy, but it is difficult to know up front whether we
will increase the size of the object or not, so we just leave it be.  I
suspect this strategy is "good enough".

Also note that we do not yet prioritize agent attention to PGs that most
need eviction (e.g., those that are full).

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