Dettach duplicated signal handler from workers

[adriangonz]

What this PR does / why we need it:

Add a post_worker_init hook to dettach a signal handler from the Gunicorn worker processes to avoid calling join() when they exit. That signal handler is attached by multiprocessing on the root process, but gets inherited by the child workers because of how fork() works.

More details can be found in the following issue: benoitc/gunicorn#1391

Which issue(s) this PR fixes:

Fixes #2094

Special notes for your reviewer:
The signal handler should still remain on the root process.

Does this PR introduce a user-facing change?:

Fixed `can only join a child process` error when shutting down a Seldon deployment.

[adriangonz]

/cc @RafalSkolasinski @axsaucedo

[seldondev]

Fri Sep 11 16:31:26 UTC 2020
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[seldondev]

Fri Sep 11 16:31:29 UTC 2020
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[adriangonz]

/test integration

[seldondev]

Fri Sep 11 16:34:31 UTC 2020
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[ukclivecox]

/approve

[seldondev]

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[seldondev]

Thu Sep 17 09:10:06 UTC 2020
The logs for [lint] [5] will show after the pipeline context has finished.
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[seldondev]

Thu Sep 17 09:10:09 UTC 2020
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[seldondev]

Thu Sep 17 12:58:45 UTC 2020
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[seldondev]

Thu Sep 17 12:58:54 UTC 2020
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