OrchestrationAsset should read open thumbs from AssetDeliveryMetadata table

[donaldgray]

MemoryAssetTracker reads data from DB and thumb sizes from S3 to populate the full OrchestrationAsset object that is used to handle incoming requests.

With introduction of AssetApplicationMetadata we can read this value from DB, rather than S3. One potential wrinkle here is either knowing that the thumb sizes are in AssetApplicationMetadata already, and falling back to S3 (see #631 for similar approach) or backfilling all thumbs (if backfilling would we want to take a similar approach to reorganising thumbs - on read from S3, write to DB and gradually populate records that way).