make the accept timeout configurable, stop using transport.AcceptTimeout

[marten-seemann]

This is the analogous change to libp2p/go-libp2p-swarm#302. Motivation: stop relying on global variables (and make dial and accept timeouts consistent).

The nicer way to configure this would be by having an option. Unfortunately, that's not possible as we don't have a constructor here.
We could refactor the code by introducing an Upgrader interface, and then unexporting the Upgrader here, while at the same time introducing a constructor.

[vyzo]

1 hour timeout!!!!!

[Stebalien]

Hm.... This works, but how about a global libp2p "config" that we plug into the dependency injection system?

1. The config would be something like: struct { TimeoutA time.Duration, Libp2pFeatureB bool, Other map[interface{}]interface{} }.
2. Config values would be set with a functional option in libp2p.
3. We'd feed the config to any transport constructors that request it.

Note: for this case, this PR is probably fine. But we do need some way to pass options down.