Make Ctrl+C or other signals set `self.done`

[WardLT]

Colmena applications do not exit cleanly on Ctrl+C because you have to kill many threads. You should be able to make the application exit more cleanly by setting the "self.done" flag of the Thinker on the first Ctrl+C, which should signal all of the threads to exit.

We need to:

1. Put the sections that sit within a try/finally block of the run function
2. Emit a logging message that indicates the signal was received
3. Wait as normal for the threads to finish