It would be nice if nighthawk_client could gracefully and quickly terminate on SIGTERM.
One way to achieve that is by:

• making the signal handling currently in effect in nighthawk_service shareable.
• add it to nighthawk_client.

upon signal reception, we could bump a counter.
we can add default termination predicates with a zero threshold to make the workers respond to that.

This gives CLI users a means to let a test run for a while and manually abort it, and still get results out.