Use congestion control when "flowctl-cc" is enabled

This should be merged only after !3113 (merged).

We previously did an if false { ... } to prevent congestion control from being used even when the experimental "flowctrl-cc" feature is enabled. This MR removes those, and also adds "flowctl-cc" features to arti-client and arti.

@nickm: Setting you as reviewer since this is small, and is a follow-up to other circuit handshake changes that you either authored or reviewed.

/cc @dgoulet