Clean up CFMetaData

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6506
Clean up OnDiskAtom and implementations

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6506
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql/AlterTableStatement.java
	src/java/org/apache/cassandra/cql/DropIndexStatement.java
	src/java/org/apache/cassandra/cql/QueryProcessor.java