ReplicatedPG: make watch timeout configurable

Fixes: #2354
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
ReplicatedPG: mark stats invalid when marking unfound lost

Fixes: #3660
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>