CqlHostDataSource::del4 () and del6() fail when given a non-existent host reservation

Neither function checks for the case of host not found, and causes the server to SIGABRT. They should both be modified to simply return true if the host does not exist. This is in keeping with our philosophy that attempting to delete an object that does not exist equates to a successful delete.

There are apparently no unit tests for this scenario and there most certainly should be. We need to verify that MySQL and PostgreSQL behave properly and have unit tests for this.