osd_types: add last_became_active to pg_stats

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: timeout watches based on last_became_active

This way a notify on an object with a single defunct watcher
won't necessarily have to wait the full timeout if the pg
has been active for a while.

Signed-off-by: Samuel Just <sam.just@inktank.com>