Cleanup: Use LiveActor to cleanup Pacto Server code

[maxlinc]

Originally proposed in #115 and #114.

LiveProviderActor - Proxies the request to a real service endpoint, either the actual provider or an out of process stub server like Mountebank or Mimic.

This could help clean up the Pacto Server code, especially, but could also be useful for normal Pacto.

Something like this might be especially useful:

Pacto::Generator 'Cloud Servers' do
  actor LiveActor, backend: 'http://localhost:3000'
  load_contracts 'contracts/cloud_servers'
end

Pacto::Generator 'Cloud Files' do
  actor LiveActor, backend: 'http://localhost:4000'
  load_contracts 'contracts/cloud_files'
end

Note that this allows Pacto to proxy different services to different endpoints (port 3000 vs 4000 above). This would it easier to use Mimic or Mountebank, which both open differnet ports.