also run setting the default preferences in a background thread

Closes #184

This closes #204

This should make it easier for people to build the project with a freshly
downloaded ADT Bundle.

We don't need two separate executors for long-running IO threads.

Tor has a controller command, TAKEOWNERSHIP, and a configuration option,
__OwningControllerProcess, that work together to ensure Tor shuts down
when the controlling process dies and/or disconnects from the control
port. By using them we can avoid creating runaway Tor processes that
have to be killed with hacks.

Thanks to the team that audited CryptoCat for this idea. :-)

Very important stuff.

This avoids a possible problem where the app expires while it's running
and every activity shows the expiry warning, leaving the user with no
way to quit.