SnippetClient does not explicitly handle self._proc in USB disconnect case.

[xpconanfan]

When the USB disconnects (without rebooting the device), self._proc essentially becomes useless.
Right now the behavior is implicitly assumed.
We should handle this better.

Discovered when doing #507

[mhaoli]

Currently, the client will set it to None when restoring the connection to the server. I think this is enough.

On the one hand, the client will no longer use self._proc. On the other hand, setting it to None will clean up the zombie processes in the system. So I think this is enough.