improve org.apache.lucene.cassandra.ACassandra.listFiles()

[jasonwee]

git show c69dce5

when we list files, sometime the child is given.
file = new ACassandraFile("/", "test", IOContext.READ, true, keyspace, columnFamily, blockSize);
file = new ACassandraFile("/foo", "test", IOContext.READ, true, keyspace, columnFamily, blockSize);

so when we list, we list beneath /test and not just parent /
so when we list, we list beneath /foo/test and not just parent /foo