Changes the security scanner code to raise exceptions now for non-successful operations. One of the new exceptions raised is MissingParentLayerException, which, when raised, will cause the security worker to perform a full rescan of all parent images for the current layer, before trying once more to scan the current layer. This should allow the system to be "self-healing" in the case where the security scanner engine somehow loses or corrupts a parent layer.
The FakeSecurityScanner mocks out all calls that Quay is expected to make to the security scanner API, and returns faked data that can be adjusted by the calling test case
Following this change, anytime a layer is indexed by the security scanner, we only send notifications out if the layer previously had a security_indexed_engine value of `-1`, thus ensuring it has *never* been indexed previously. This will allow us to change to version of the security scanner upwards, and have all the images be re-indexed, without firing off notifications in a spammy manner.
Fixes#1301
- Ensures that the worker uses pagination properly
- Ensures that the worker handles failure as expected
- Moves marking the notification as read to after the worker processes it
- Increases the number of layers requested to 100
