New class MySQLConnectionCheck, to check whether MySQL servers are alive, uses threads
Changes in DataDistribution:
- new field: String[] nodes, initialised in constructor
- changed constructor to check for nodes that are alive, using new method nodesAlive
- renamed getNodes to getMySQLNodes. New method getNodes just returns the new field String[] nodes
DataDistributer: renamed calls to DataDistribution's getNodes to getMySQLNodes