E2E benchmarks for celery and Kafka

[mariusandra]

We have benchmarks (#5 and #41), yet they just measure the performance of the VM, yielding results up to 100k events/sec under specific loads (simple plugin + batches of 100)

What we don't have are E2E tests that take the overhead of Celery and Kafka into account.

Possibly related: #55

[mariusandra]

Closed with #144