Restore ofy keys in GracePeriod objects (#846)

* Restore ofy keys in GracePeriod objects

Restore the ofy keys when loading GracePeriod object from SQL.  There's no
clear way to do this using the normal approach (fix-up during a PostLoad
method) because fixups to these violate immutability after hibernate has
already obtained their hash values.  Instead, we force reconstitution of the
ofy keys in all public methods that access them (including equals() and
hashCode()) so that they can be generated before an invalid hash is generated.

As part of this change, convert the GracePeriod id from an autogenerated
sequence to a UUID allocated from ObjectifyService and enhance ImmutableObject
to allow it to exclude certain fields from hash/equals and print.

The ImmutableObject enhancements are necessary because we compare grace
periods against locally created test objects in a number of unit tests and
there's no way this can work with GracePeriods loaded from SQL currently, as
they will have an identifier field generated from the database and the test
objects will have an identifier field of null (or a new unique value, after
this change).

Removing autogeneration from GracePeriod ids ended up being likely not
strictly necessary for this change (it was a consequence of an earlier
iteration).  However, it does alleviate the problem of mutation of an
immutable object after creation and is more in line with how we've decided to
allocate other identifiers.

* Changed needed after rebase.

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

@@ -383,9 +383,11 @@
     );
 
     create table "GracePeriod" (
-       id  bigserial not null,
+       id int8 not null,
         billing_event_id int8,
+        billing_event_history_id int8,
         billing_recurrence_id int8,
+        billing_recurrence_history_id int8,
         registrar_id text not null,
         domain_repo_id text not null,
         expiration_time timestamptz not null,