Allow explicit close of output channels

[mariomac]

There is a maximum in golang that says:

Senders close; receivers check for close

However, output channels in pipeline stages are automatically closed. That causes that some asynchronous operations require to implement extra synchronization mechanisms to double check that the output channel is not close.

While we can keep automatic close of channels by default, we can provide an extra graph/node builder argument to allow explicit closing of output channels.