dbmate should roll the transaction back on sigint

[wkalt]

I think today there is a shutdown race that sometimes causes a rollback, but to make it consistent I think we need a signal handler in here:
https://github.com/amacneil/dbmate/blob/master/pkg/dbmate/db.go#L188,L203

How about the first sigint triggers the rollback (and subsequent exit), and the second one does an immediate exit?

[wkalt]

#99 would change the location of the signal handler

[amacneil]

Sounds good. I like the behavior of graceful shutdown on first signal and hard exit on second.

We should probably handle SIGTERM the same way, I don't know what our current behavior is.