PoC 2: Two links

Proof of Concept 2:
Solve internal flow control:
```
AMQP writer proc <--- sesssion proc <--- queue proc
```
by adding a second link:
AMQP client <---link--> session proc <---link---> queue proc
```

The session proc limits how many credits it tops up
on the second link to the queue proc.
If the AMQP client chooses a huge value on the first link,
the session proc will top up credit in batches on the second link.