restr field stored as hardcoded "OPEN" in MongoDB — restriction status not persisted, only resolved at query time

[NikolaosSokos]

The availability MongoDB collection always stores restr: "OPEN" for every
record, regardless of the actual restriction status of the stream. This has been
the case since the first version of views/main.js.

The real restriction status is resolved at query time in _apply_restricted_bit()
(apps/wfcatalog_client.py), which cross-references the segment's SEED ID against
the Redis-cached station inventory from ws-station.

[jschaeff]

For the moment, we need to live with this design.

[petrrr]

This field seems to be just a placeholder, though do not really understood why it needs to be there as it could be added on the fly. We might consider getting rid of this field in the future.