Make PremiumList.labelsToPrices "insignificant" (#1167)

* Make PremiumList.labelsToPrices "insignificant"

Add the ImmutableObject.Insignificant annotation to labelsToPrices and also
mark it as Transient.  In order to do lazy-loads on this field, we need to do
so explicitly: doing otherwise breaks the immutability contract and prevents
detaching the object upon load.

Note that this is an expedient solution to this problem, but not the optimal
one.  Ideally, the disassociation between PremiumList and its PremiumEntry's
would be more explicit.  However, breaking labelsToPrices out would at minimum
require reworking the Create/UpdatePremiumList commands, which currently rely
on passing around a self-contained PremiumList object, both from the parser
interfaces and to the database.

If this approach is acceptable, we can apply it to ReservedList and ClaimsList
as well (though it may be easier to break the association in those cases).

* Fix premium list "delete" to support a test

* Fix a few more tests

* Changes for review (updated javadocs)

* Minor fixes

* Updated getLablesToPrices() comment

* Format fixes, fixed PremiumEntry interfaces

PremiumEntry can now be SQL only.

db/src/main/resources/sql/schema/db-schema.sql.generated

@@ -862,11 +862,6 @@ create index spec11threatmatch_check_date_idx on "Spec11ThreatMatch" (check_date
        foreign key (domain_repo_id, domain_history_revision_id) 
        references "DomainHistory";
 
-    alter table if exists "PremiumEntry" 
-       add constraint FKo0gw90lpo1tuee56l0nb6y6g5 
-       foreign key (revision_id) 
-       references "PremiumList";
-
     alter table if exists "RegistryLock" 
        add constraint FK2lhcwpxlnqijr96irylrh1707 
        foreign key (relock_revision_id)