* moved the locking to a separate function "acquire_lock"
* changed $[ ] to $(( )), for consistency with the rest of the file
* taking the lock only after checking all other preconditions (they are non-mutating and thread-safe)
flock is not supported on MacOS.
`mkdir` locks have a drawback: they are not cleaned up automatically. If the lock owner crashed before cleaning up the lock, the directory will stay in the filesystem and the lock will be never acquired by someone else. To avoid that, we create temporary locks (the lockdir name changes every 100 seconds). We grab two lock (N and N+1) to avoid the case where process A grabs lock N and process B grabs lock N+1 and both enter the critical section.
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.
