Even with cdnPattern: source, dfget connects to the downloadPort

[jayme-github]

Ⅰ. Issue Description

When a supernode is configured with cdnPattern: source, dfget clients should connect the source directly instead of using the supernodes CDN (via a webserver listening on downloadPort) - at least this is my understanding.

Ⅱ. Describe what happened

When downloadPort on the supernode is not configured or pointing to a not open port, dfget clients hang when trying to load a file. tcpdump shows that dfget is connecting to the downloadPort regardless of the cdnPattern configuration but does not send a request. It is sufficient so set downloadPort == listenPort on the supernode to word around this.

Ⅲ. Describe what you expected to happen

dfget should not connect to the downloadPort of the supernode.

Ⅳ. How to reproduce it (as minimally and precisely as possible)

1. Configure supoernode cdnPattern: source and don't listen on downloadPort
2. Run dfget

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• dragonfly version: v1.0.6
• OS (e.g. from /etc/os-release): Debian 9
• Kernel (e.g. uname -a): 4.19.0-0.bpo.14-amd64
• Install tools: docker 18.06.3