Faster shutdown process for stacks-node

[wileyj]

Currently, it can take stacks-node anywhere from seconds to just short of 20 minutes to complete a full shutdown.

It's often not clear after ctrl-c, or kill pid why the shutdown is taking as long as it does - which leads to potential corruption of the chainstate via kill -9 pid.

Can the shutdown sequence be improved so that even kill -9ing would not lead to a corrupt chainstate, or at a minimum the logs may reflect why the node isn't shutting down as fast as expected?