fix memory leaks when trowing exceptions in cql_exchange
Currently the CqlExchange::executeSelect function returns an AnyArray of dynamically allocated objects. After each execution, the array is iterated and all objects are added to shared_pth objects. If there is an exception thrown in the CqlExchange::executeSelect function, the AnyArray vector is released causing memory leak because the stored pointers are never deleted.
This issue is resolved by github pull request: https://github.com/isc-projects/kea/pull/112