Write a performance test with an empty protocol

[fjarri]

To have an idea how much time the whole framework consumes, and where it is spent, we need a performance test running a several-round protocol that does nothing but sending and receiving empty messages. Maybe with a hundred nodes.

Note that:

• we already have testing signers/verifiers which are very fast, but one will also have to provide a custom serializer and a digest (in the Protocol impl) which do as little work as possible.
• some time will be necessarily spent in run_sync(). How much? Will need to profile.