Differentiate between inserts and updates in flows (#2846)

Updates (AKA merges) run an extra SELECT statement to figure out if the
resource exists so that it can merge the entity into the existing object
in Hibernate's schema. When we're inserting new rows (such as new poll
messages or resource creates), we know that we don't need to do that
merge. Doing this should save us some SELECT statements (this has borne
out to be the truth in alpha)

core/src/test/java/google/registry/flows/custom/TestDomainCreateFlowCustomLogic.java

@@ -40,8 +40,9 @@ public class TestDomainCreateFlowCustomLogic extends DomainCreateFlowCustomLogic
               .setMsg("Custom logic was triggered")
               .build();
       return EntityChanges.newBuilder()
-          .setSaves(parameters.entityChanges().getSaves())
-          .addSave(extraPollMessage)
+          .setInserts(parameters.entityChanges().getInserts())
+          .addInsert(extraPollMessage)
+          .setUpdates(parameters.entityChanges().getUpdates())
           .setDeletes(parameters.entityChanges().getDeletes())
           .build();
     }