osd/scrub: improve scrub target dumps

osd/scrub: improve scrub target dumps

Following the changes to the internal structures used for
scrub scheduling, improve the code handling external queries
and dumps, so that:
- common code is used when creating the JSON lines for scrub targets;
- the data dumped more closely matches the internal state;

Specifically:
- the scrub-queue dump ('tell osd.1 dump_scrubs') now includes
  whether the target is active (being scrubbed) or not.
- the scrubber dump (the ',scrubber' object in 'pg <pgid> query'
  output) now also dumps the two scrub targets.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>