osd: Interim backfill changes

Make peer_backfill_info a map which holds a
BackfillInterval for all backfill targets.
Initially see if recover_backfill() can just backfill
the first one and mark them all finished.

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: Implement multiple backfill target handling

Fixes: #5858

Signed-off-by: David Zafman <david.zafman@inktank.com>