Migrate key_alias->key_aliases on every schema migration (1.2)

Another follow-up to 5800

src/java/org/apache/cassandra/db/DefsTable.java

@@ -341,6 +341,10 @@ public class DefsTable
         for (RowMutation mutation : mutations)
             mutation.apply();
 
+        // Must be called after each schema pull and not just on startup to guarantee the migration.
+        // See CASSANDRA-5800 comments for the details.
+        SystemTable.migrateKeyAlias();
+
         if (!StorageService.instance.isClientMode())
             flushSchemaCFs();
 

src/java/org/apache/cassandra/db/SystemTable.java

@@ -184,11 +184,10 @@ public class SystemTable
         migrateKeyAlias();
     }
 
-
     /**
      * 1.1 used a key_alias column; 1.2 changed that to key_aliases as part of CQL3
      */
-    private static void migrateKeyAlias()
+    public static void migrateKeyAlias()
     {
         String selectQuery = String.format("SELECT keyspace_name, columnfamily_name, writetime(type), key_aliases, key_alias FROM %s.%s",
                                            Table.SYSTEM_KS,