ssh-proxy.scm: Migrate to Guile-SSH 0.3.  Add additional traces.

Actually it fixes the problem with periodical ping, because this
problem was fixed in Guile-SSH 0.3.

* proxies/ssh-proxy/ssh-proxy.scm (handle-send-message): Make it work
  with Guile-SSH 0.3.  Add additional traces.
* README: Add Guile-SSH 0.3 to dependencies.