Fix for prepared statements in multi-node environments

Prepared statements in multi-node environments are failing. After switching the cassandra node, the cpp-driver tries to execute the prepared statement instead of preparing it. As a result, the prepared statement is not available on the cassandra node and the prepared statement can not be executed.

For additional information, see the discussion at the user mailing list: https://groups.google.com/a/lists.datastax.com/forum/#!topic/cpp-driver-user/d0TngHHDIOk