Refactor list_to_streams and create_streams_if_needed to take a list
of dictionaries, instead of a list of stream names. This is
preparation for being able to pass additional arguments into the
stream creation process.
An important note: This removes a set of validation code from the
start of add_subscriptions_backend; doing so is correct because
list_to_streams has that same validation code already.
[with some tweaks by tabbott for clarity]
8461cc411e