Fix errors causing some tests in PartitionTest to fail.

[daniepin]

When trying to call join() on threads in Partitioner::startCoordinated() C++ will sometimes throw an invalid_argument exception as some of or all of the threads might be default constructed and therefore not joinable. This happens because the threads are only assigned a function if a given condition is met. This PR fixes this by checking if a thread is joinable before calling join() on the thread, which subsequently fixes and passes PartitionTest.test_1, PartitionTest.test_3, PartitionTest.test_5 and PartitionTest.test_7.

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[ZigRazor]

Good Work!