Set cluster cookie as label on workers

[omus]

I was thinking about having some kind of unique identifier to determine the workers which are associated in a cluster after making the worker_prefix change in #104. One area of concern is that the worker_prefix could be changed between calls to addprocs. That made me remember that cluster_cookie() is what Julia internally uses to ensure that the manager and worker are a part of the same group so this seemed like a good choice to have for an additional label.

I'll mention that you can modify the cluster_cookie by calling it and providing a string. However, I very much expect things would break if you change this between calls to addprocs but I could be wrong on that.

[codecov[bot]]

Codecov Report

Merging #106 (08a611f) into main (e8a90b3) will increase coverage by 0.20%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   65.64%   65.85%   +0.20%     
==========================================
  Files           4        4              
  Lines         163      164       +1     
==========================================
+ Hits          107      108       +1     
  Misses         56       56              

Impacted Files	Coverage Δ
src/native_driver.jl	11.66% <0.00%> (ø)
src/pod.jl	97.61% <100.00%> (+0.02%)	:arrow_up:

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