ceph: put unused osd connections on lru

Instead of removing osd connection immediately when the
requests list is empty, put the osd connection on an lru.
Only if that osd has not been used for more than a specified
time, will it be removed.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>