Add healtcheck mechanism for local endpoints

[mbyczkowski]

Wormhole client should have configurable settings to periodically check that the local endpoint is up and running. At the very least we want to ping the local endpoint periodically and make sure we can connect. Ideally, there would be a way to define a URL that could be hit with expected output.

If local endpoint is not healthy, the wormhole client should shut down the tunnel and keep trying the local endpoint until it can connect again, and then re-establish a new session with wormhole server.