Move producers polling into a single thread

[mensfeld]

With #56 and #119 there will be much bigger need for one producer per thread.

This cannot be done in an efficient way if we create a thread per instance hence this proposal.

In order to make sure we do not change the behaviour for multi-threaded single producer and we ensure the consistent experience of deliveries as we do now the proposition is as follows:

1. Create a single thread for all producers
2. Look into fiber scheduler to be able to wake up in an async manner OR investigate some sort of improvements over current polling mechanism and the CPU usage of a 1ms polling (dunno yet)

The current threading model is def. not sufficient for multi-threaded operations and due to the producer "self managing" polling it seems this needs to be done at this layer