Use Disable() and Exit() instead of Stop()

The `github.com/cmars/conflux` package has a `Stop()` function that
calls into `Disable()`, and then cleans up everything and stops the
gossip server. However, this function is buggy.

Since we just want to exit anyway, change this into a `Disable()`
followed by an `Exit()`. Memory leak, gossip server leak, but it seems
to function.