Delete old triggers from cache when training is over

[MaxiBoether]

In issue #124, we deregister an entire pipeline and can then delete all cached triggers from the Selector (by deleting the Selector object itself).

With this issue, we want to enable that the trainer server can say "I finished training on trigger 42, you can delete the data from cache" to the selector, to clean up memory.

Should be addressed together with #308.