Add partially shared Pulsar objects

[vallis]

This PR adds Pulsar methods deflate(), inflate(), and destroy() that allow sharing Pulsar objects among processes on the same host/node.

The workflow is as follows:

• A prep-chef script loads all pulsar objects, deflates them, pickles them.
• All sous-chef scripts load the pickles, inflate the objects, use them.
• A kitchen-cleaner loads the pickles, destroys the objects, deletes the pickles.

This functionality requires Python 3.8 (so it is not tested in 3.6 and 3.7). It also requires dedicated testing on our production clusters.

[codecov[bot]]

Codecov Report

Merging #289 (e92596b) into master (9ad4a29) will increase coverage by 0.19%. The diff coverage is 98.52%.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   86.76%   86.95%   +0.19%     
==========================================
  Files          12       13       +1     
  Lines        2819     2868      +49     
==========================================
+ Hits         2446     2494      +48     
- Misses        373      374       +1     

Impacted Files	Coverage Δ
enterprise/pulsar_inflate.py	96.15% <96.15%> (ø)
enterprise/pulsar.py	91.96% <100.00%> (+0.50%)	:arrow_up:

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[vallis]

Coverage fails on 3.6 and 3.7 because of the new shared-memory code. Should be able to exclude it along the lines of https://sobolevn.me/2020/02/conditional-coverage

[AaronDJohnson]

I have tested this branch with both multiple serial tasks running on the same node and with MPI tasks running on the same node on a local cluster. Both versions are working as intended when using a Slurm scheduler.