Fixes the case where multiple tmux sessions call auto-save at the same time, which occasionally results in multiple instances of save_all() running in parallel and causing issues like #3 and tmux-plugins/tmux-resurrect#294.
The sequence in main() is:
1. Check enough_time_since_last_run_passed
2. Save
3. Update last_save_timestamp.
The race here is:
* process A finishes step 1 and is busy with step 2. The timestamp is not updated yet.
* process B comes to step 1, sees the old timestamp and proceeds to step 2, too.
v9v