Parallel printing for multiple sources

[andreasknoepfle]

Just adding this as an issue, to not get lost :)

#194 (comment)

---

Implementation ideas:

• Simple but meh: Spawn a task for each and await them all
• Reasonable but expensive: Rework the session pool to yield a session and add a way to enqueue multiple protocols from a single process (so replace GenServer.call in Channel)