Summary:
Use sigaction to register signal handlers.
This is preferred over `signal` function, according to `man 3p signal`.
Add global sig_atomic_t flag when shutting down.
Block other signal handlers when shutting down.
Reviewers: mislav.bradac, mferencevic
Reviewed By: mferencevic
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D943