Duplicate entries in TakeoffLanding and Logbook table

[Meisterschueler]

Duplicate entries can be caused by a compute task (compute_takeoff_landings or compute_logbook) running more than once. Possibilities to fix it:

• http://docs.celeryproject.org/en/latest/tutorials/task-cookbook.html#ensuring-a-task-is-only-executed-one-at-a-time
• https://pypi.python.org/pypi/celery_once/0.1