Added task execution to hold information for one single try

[wachterjohannes]

No description provided.

[wachterjohannes]

@alexander-schranz @chirimoya would that be enough for your project? i would like to release that in this way.

For a future release i will refactor maybe the task to combine frequent task and use the executions to hold informations for each run and hold the "frequency" in form of a cron-expression in the task itself. this frequency would be null if it will be executed only once. the start and end date would that used as:

• not execute before (start-date)
• not execute after (end-date)

which would be a better solution than the current one.

[alexander-schranz]

cant imagine how it is implemented in the entity. if i look at the tasks table see there task_key, started, finished its ok.