nomad uses flock for data_dir

[jazzyfresh]

followup to #6348 #6348 (comment)

We should be using flock (https://github.com/gofrs/flock) to lock access to the data directory when a Nomad process accesses it. If another Nomad process tries to access the same data directory, it should fail.

[stale]

Hey there

Since this issue hasn't had any activity in a while - we're going to automatically close it in 30 days. If you're still seeing this issue with the latest version of Nomad, please respond here and we'll keep this open and take another look at this.

Thanks!