Migrate fjage.c to use sc-queue

[notthetup]

The fjåge C gateway uses an custom generic queue implementation to store messages https://github.com/org-arl/fjage/blob/master/gateways/c/fjage.c#L243

We have had some issues with the queue implementation before. It might be safer to rely on an external well-tested queue implementation instead of re-inventing the wheel.

https://github.com/tezc/sc/tree/master/queue is a generic and clean and well-tested open-source implementation of queue. We can consider moving to that.