Proper signal handling for backup and restore operations

[bison]

It looks like long-running backup and restore operations don't respond to interrupt signals, i.e. I can't Ctrl-C to abort. One issue was that these commands were not using the context from cobra that gets the cancelation on SIGINT, but even with a fix for that in #285, the signal seems to be ignored.

Doe the bulk import and export operations not handle context cancelation maybe? Is it safe to cancel a bulk import midway through?