update_test_lifecycle may fail because datetime.strptime() is not thread safe

The update_test_lifecycle.py script fetches the test history in multiple threads.
In these threads the code calls the test_failures module which uses the datetime.strptime() method.
As filed in https://bugs.python.org/issue7980, that method does not import _strptime.py in a thread safe way.
This can lead to the following error: AttributeError: 'module' object has no attribute '_strptime'.

We need to implement a workaround: either calling the strptime() method or importing the _strptime module explicitly before starting the threads.