Add nfs_threads command

[don-brady]

Display NFS thread usage info along with NFS I/O context (for both v3 and v4 clients).

Output Sample:

 packets  sockets threads threads metadata  read    read  write   write
 arrived enqueued   woken   used     calls  iops thruput   iops thruput
    4589        0    4589     25        16   273   3.6MB    212   2.6MB
    4735        0    4735      8         1   287   3.8MB    212   2.7MB
    4693        0    4693     10         0   280   3.7MB    216   2.7MB
    4625        0    4625     15         0   278   3.7MB    212   2.6MB
    4687        0    4687      7         1   285   3.8MB    210   2.6MB
    4701        0    4701     12         0   285   3.8MB    215   2.7MB

Testing
http://selfservice.jenkins.delphix.com/job/devops-gate/job/master/job/appliance-build-orchestrator-pre-push/2649/
Confirmed that the added command is present in the built image and runs.

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build was run locally and any changes were pushed
• flake8 has passed locally and any fixes were made for failures

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• Bugfix
• Feature
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• Refactoring (no functional changes, no api changes)
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[don-brady]

Looks like flake8 is applying python2.7 syntax checking to my python3 script.

[sebroy]

Looks like flake8 is applying python2.7 syntax checking to my python3 script.

@don-brady take a look at .github/workflows/flake8.yml. In order to get this to work appropriately, we'll need to change that action to have separate jobs for Python 2.7 and Python 3 checking, with the jobs applying to the specific files or directories that are relevant to that Python version.

[sebroy]

Either that or we bite the bullet and convert everything over to Python 3 in this repo and change the action to do Python 3 checking across the board. I'd love to see that too. This is already being reviewed in #9.