bug(engine): collector cannot be used after a SHUFFLE step

[leosmerling-hopeit]

There might be cases where in a list of steps, a collector step is needed for async concurrent execution of tasks.
Collector allows these, but it was intended only to work as the initial step for an event.
There is a case though, that sequential steps need to be executed before the collector, and a SHUFFLE could be
interleaved to distribute the execution. In this case the engine fails to call the collector with the deserialized payload
from a stream.