PG, OSD: reject backfills when an OSD is nearly full

PG, OSD: reject backfills when an OSD is nearly full

Reject backfills when an OSD reaches a configurable full ratio. Retry
backfilling periodically in the hopes that the OSD has become less full.

This changeset introduces two configuration options for dealing with
this: osd_refuse_backfill_full_ratio and osd_backfill_retry_interval.

We also introduce two new state transitions in the PG's Active state.

Signed-off-by: Mike Ryan <mike.ryan@inktank.com>