Changes the asset key storage to be an JSON array instead of a custom
delimited string representation. Relaxes the AssetKey parameters to be
permissive instead of raising an Exception.
Could also consider a schema migration and supporting both a structured json
field as well as the legacy delimited field, so we don't have to do these OR sql
clauses (and deduping). That has a downside of forcing a migration, and so would
need to delay this until the 0.10.0 launch.