ReservedKeywords class is missing some reserved CQL keywords

The CQL keywords DEFAULT, UNSET, MBEAN and MBEANS (introduced by CASSANDRA-11424 and CASSANDRA-10091) are neither considered unreserved keywords by the ANTLR parser, nor included in the ReservedKeywords class.

The current parser behaviour is considering them as reserved keywords, in the sense that they can't be used as keyspace/table/column names, which seems right:

cassandra@cqlsh> CREATE KEYSPACE unset WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1};
SyntaxException: line 1:16 no viable alternative at input 'unset' (CREATE KEYSPACE [unset]...)

I think we should keep considering these keywords as reserved and add them to ReservedKeywords class.