Fix handling of concurrent directory creation failure

patch by Ryan Fowler; reviewed by Aleksey Yeschenko for
CASSANDRA-6459
Merge branch 'cassandra-1.2' into cassandra-2.0
Merge branch 'cassandra-2.0' into trunk
Fix test build
Merge branch 'cassandra-1.2' into cassandra-2.0
Merge branch 'cassandra-2.0' into trunk
remove dead fields
Merge branch 'trunk' of http://git-wip-us.apache.org/repos/asf/cassandra into trunk