mon: fix quorum_age() regression

mon: fix quorum_age() regression

We need to cast this to chrono::seconds or else it will overload the int
return type.

Partially reverts 1a9aab62b615830107ee28866630f043e1a01a35: the quorum_age()
users want seconds explicitly. The status caller reverts back to chrono
types so that it can be passed to timespan_str().

Fixes: 1a9aab62b615830107ee28866630f043e1a01a35
Signed-off-by: Sage Weil <sage@newdream.net>
(cherry picked from commit 68cfe785ccbc9ebd2a28492327142bb5878181a4)