Add prepared statements to CassandraCQLClient.

Prepared Statements added for read, update, scan, insert and delete. Statements for CRUD are stored in maps where the queryString is the key, for scan only a single prepared statement is required as the fields that are scanned do not vary. 

The final modification for the client is the removal of `cluster.close();` in the cleanup as it's called prematurely leading to `NoHostAvailableException ` exceptions during query execution.