Multiple bugs with apps

[nkryuchkov]

1. Visor cannot run more than one app.

Describe the bug
If visor config has more than one app, the second and the next ones cannot be started due to the following error:
ERROR [app_skysocks]: error serving RPC error="listen unix /tmp/appserverA.sock: bind: file exists"

Environment information:

• OS: Mac OS X
• Platform: Darwin 18.7.0 x86_64

Steps to Reproduce
Steps to reproduce the behavior:

1. Add the skysocks app config from integration/proxy/nodeA.json to integration/generic/nodeA.json as the second app.
2. Start some nodes using the make integration-run-generic command of the skywire-services repository.
3. Check NodeA pane in tmux.
4. You will see the error, the first app will be run, the second won't be (you may check ps aux).

Actual behavior
More than one app on a visor does not start.

Expected behavior
Multiple apps on visor start with no error.

Possible implementation
Hints to fix the bug.

2. Apps cannot be restarted

Describe the bug
If an application is stopped by calling the PUT /api/visors/{pk}/apps/{app} API endpoint with the {"status": 0} payload, the app is not stopped and then started.

Environment information:

• OS: Mac OS X
• Platform: Darwin 18.7.0 x86_64

Steps to Reproduce
Steps to reproduce the behavior:

1. Start some nodes using the make integration-run-generic command of the skywire-services repository
2. Call PUT /api/nodes/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7/apps/skychat with {"status": 0} as content, to stop the skychat app.
3. Check NodeA pane in tmux.
4. You will see the error, the app will not be stopped (you may check ps aux).

Actual behavior
An app cannot be stopped.

Expected behavior
An app can be stopped and then started.