ask tor to leave dormant mode

In #28624 (moved), tor was changed to remember dormant state across restarts. Since Tor Browser in nearly every case is started by a real person who expects to access one or more websites after starting the browser, it makes sense to ask tor to come out of dormant mode every time we start tor.

This can be done by issuing a SIGNAL ACTIVE control port command, although we could also ask the network team to add a torrc option to get this behavior.

For more background, see the following tor-dev message thread: https://lists.torproject.org/pipermail/tor-dev/2018-December/013588.html