Peer can deadlock on task completion

[MichaelDrogalis]

Reproduced in core.async plugin tests with a high number of virtual peers. Sometimes, closing a peer will block as it tries to flush its pipeline. The pipeline will block on reading from an ingress queue. This queue should always provide the sentinel value. Something is hanging on to the sentinel as a consumer and never committing it back to the queue, hence the hang.

[MichaelDrogalis]

Tentative fix on 33ff91e.

[MichaelDrogalis]

Fixed in release 0.3.3.