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.
- this caused issues with the restore missing some processes and putting
others in the wrong place.
- I think this may need some work with the tmux-resurrect plugin to fix if auto
restore is to be used.
Do not count `tmux source-file .tmux.conf` as a tmux process when checking if
other tmux server is running.
Previously, this caused interpolation command not to be inserted into
`status-right` because `tmux source-file` was falsely detected as another tmux
server.
