notmuch new: Don't prevent database upgrade from being interrupted.

notmuch new: Don't prevent database upgrade from being interrupted.

Our signal handler is designed to quickly flush out changes and then
exit. But if a database upgrade is in progress when the user
interrupts, then we just want to immediately abort. We could do
something fancy like add a return value to our progress_notify
function to allow it to tell the upgrade process to abort. But it's
actually much cleaner and robust to delay the installation of our
signal handler so that the default abort happens on SIGINT.