Add unique constraints on domain_hosts (#911)

* Add unique constraints on domain_hosts

Add unique constraints on DomainHost (child of DomainBase) and
DomainHistoryHost (child of DomainHistory). DomainHost is non-entity
embedded object and Hibernate does not define indexes automatically.

This should improve read and write performance of the parent entities.

core/src/main/java/google/registry/model/domain/DomainBase.java

@@ -75,7 +75,9 @@ public class DomainBase extends DomainContent
   }
 
   @ElementCollection
-  @JoinTable(name = "DomainHost")
+  @JoinTable(
+      name = "DomainHost",
+      indexes = {@Index(columnList = "domain_repo_id,host_repo_id", unique = true)})
   @Access(AccessType.PROPERTY)
   @Column(name = "host_repo_id")
   public Set<VKey<HostResource>> getNsHosts() {

core/src/main/java/google/registry/model/domain/DomainHistory.java

@@ -96,7 +96,14 @@ public class DomainHistory extends HistoryEntry implements SqlEntity {
   // TODO(b/166776754): Investigate if we can reuse domainContent.nsHosts for storing host keys.
   @Ignore
   @ElementCollection
-  @JoinTable(name = "DomainHistoryHost")
+  @JoinTable(
+      name = "DomainHistoryHost",
+      indexes = {
+        @Index(
+            columnList =
+                "domain_history_history_revision_id,domain_history_domain_repo_id,host_repo_id",
+            unique = true),
+      })
   @Column(name = "host_repo_id")
   Set<VKey<HostResource>> nsHosts;