Add ZED_STATELESS env var, for suppressing persistence

Use this env var in the start-local-collaboration script to make
the behavior more predictable.

script/start-local-collaboration

@@ -36,6 +36,7 @@ position_1=0,0
 position_2=${width},0
 
 # Authenticate using the collab server's admin secret.
+export ZED_STATELESS=1
 export ZED_ADMIN_API_TOKEN=secret
 export ZED_SERVER_URL=http://localhost:8080
 export ZED_WINDOW_SIZE=${width},${height}