Add an index on Host.host_name column (#1510)

* Add an index on Host.host_name column

This field is queried during host creation and needs an index to speed
up the query.

Since Hibernate does not explicitly refer to indexes, we can change the
code and schema in one PR.

core/src/main/java/google/registry/model/host/HostResource.java

@@ -34,6 +34,9 @@ import javax.persistence.AccessType;
 @ReportedOn
 @Entity
 @javax.persistence.Entity(name = "Host")
+@javax.persistence.Table(
+    name = "Host",
+    indexes = {@javax.persistence.Index(columnList = "hostName")})
 @ExternalMessagingName("host")
 @WithStringVKey
 @Access(AccessType.FIELD) // otherwise it'll use the default if the repoId (property)