listen on $TCHANNEL_PORT if it's available

[blampe]

If we're not explicitly provided a port to listen on, but we have a TCHANNEL_PORT environment variable available, then we should prefer to listen on that port instead of dynamically picking one.

[Raynos]

This belongs in one line init.

We need to author and support one line init.

The listen() call takes mandatory port and ip.

The cheapest change would be to update weaver template with copy+pasted ENV parsing code.

[blampe]

We want to be able to pick this up with a client lib bump. Do you mean people are explicitly passing in port 0?

[Raynos]

Explicit zero or static.

Everyone passes in explicit ip from "my-local-ip" library.

[blampe]

This could still be done all in the client library -- if we're passed a port of 0 but this environment variable is set, then prefer the environment's port.

[Raynos]

I think this kind of global state reading belongs in one-line-init.

It's unreasonable for the channel.listen() call to read global state.

We should get a one line init going.