This solves problems where users couldn't shut down sessions while
locators or build tasks are running.

I renamed `debugger::Session::Mode` enum to `SessionState` to be more
clear when it's referenced in other crates. I also embedded the boot
task that is created in `SessionState::Building` variant. This allows
sessions to shut down all created threads in their boot process in a
clean and idiomatic way.

Finally, I added a method on terminal that allows killing the active
task.

Release Notes:

- Debugger: Allow shutting down debug sessions while they're booting up