By enabling the `hs-pow-full` feature

This commit does not implement the actual POW challenge
yet, just a dummy. It does provide a working protocol upgrade though.

A custom `InboundConnectionUpgrade`/`OutboundConnectionUpgrade`
implementation that wraps `noise` (authentication + encryption)
but before running the `noise` handshake the listening
party sends a proof of work challenge, to which the dialing
party must respond correctly.
Else the connection is terminated before it is setup.

This will be a breaking change.