common-fs2: create the source and sink at once so that they can share clients

[benjben]

At the moment the creation of the source and the sinks are independent and as a consequence they can't share resources (e.g. clients).

For instance for Kinesis a client is initialized for each sink, whereas it could be initialized once and shared. Same for Kafka and RabbitMQ.

Creating the source and sinks at the same time would make it possible for them to share resources.