osd/ReplicatedPG: kickstart the agent if scrub stats become valid

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: dump agent state on pg query

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: warn if invalid stats prevent us from activating agent

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