We have already merged #1390 where we moved `ignore_errors` before sampling.

Background for the order change:
We want to update the session for dropped events in case the event is dropped by sampling. Events dropped by other mechanisms should not update the session. See getsentry/develop#551

Now we would like to discuss if we can simply move sampling after `before_send` and `event_processor` and update the session right before sampling.
What are implications of changing this?
* How does this affect session count and session crash rate?
* Will this have a negative effect on performance as `before_send` and `event_processor` will now be executed for every event instead of only being executed for sampled events? Developers may have fine tuned their sample rate for a good performance tradeoff and now we change. Also developers can supply their own implementations for both `before_send` and `event_processor` on some SDKs so we have no way of predicting performance I'm afraid.
* We are uncertain why a developer chose to drop an event in `before_send` and `event_processor`:
** Was it because they want to ignore the event - then it shouldn't update the session
** Or was it to save quota - then it should update the session

Please feel free to optimize the code this is just to start the discussion.