db/schema_tables: Add schema_functions and schema_aggregates tables

The 3.0.3 Java driver, for example, search for the tables and fails when
we advertise Cassandra 2.2 version from Scylla.

db/schema_tables.cc

@@ -2175,7 +2175,7 @@ data_type parse_type(sstring str)
 
 std::vector<schema_ptr> all_tables() {
     return {
-        keyspaces(), columnfamilies(), columns(), triggers(), usertypes(), /* Not in 2.1.8 functions(), aggregates() */
+        keyspaces(), columnfamilies(), columns(), triggers(), usertypes(), functions(), aggregates(),
         views(),
     };
 }