MPI: Bugfix in initialization for rare corner case

There was a bug when one reader had only empty ACQt files in its
sublist. That caused the consumer to wait indefinitely for the broadcast
of the timestamps.

Solution:

Go back from broadcasting timestamps to simple send and receive. The
operation only happens once in the initialization phase and is therefore
not time critical. Simple send and receive provides the advantage that
the consumer can probe for the message asynchronously.

Consumers now alternatingly check for:

- a message with timestamps
- a message that they should quit listening

Now consider the two scenarios:

a) there are entries in the list of ACQt files
- reader will return true from InitializeACQtFiles and consequently send
the timestamps
- consumers will receive the timestamps and return true as well.

b) there are no entries
- reader will send message to stop listening and return false
- consumer will receive message to stop listening and also return false
Removed debug output.