Add option 'worker_shutdown'

[sirihansen]

The shutdown option in the child specs for each worker is hardcoded to 5000 ms. If something is slow during shutdown, this may add up to quite a lot depending on the number of workers. In my case I want to allow the workers to empty their inboxes on controlled termination so I don't want to use the default brutal_kill for the supervisor, but I also don't want to allow too much time if something hangs or is slow during this cleanup.

This PR includes:

• The new option worker_shutdown which can have the value brutal_kill | timeout()
• Add short description of option pool_sup_shutdown to README.md

[codecov]

Codecov Report

Merging #171 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   96.35%   96.36%           
=======================================
  Files          10       10           
  Lines         439      440    +1     
=======================================
+ Hits          423      424    +1     
  Misses         16       16           

Impacted Files	Coverage Δ
src/wpool_process_sup.erl	100.00% <100.00%> (ø)

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[elbrujohalcon]

@sirihansen I'm inclined to merge this PR, but I'll wait in case you want to fix the description in the readme as you proposed.

[elbrujohalcon]

No need for squashing. Thanks, @sirihansen :)
I'll merge and generate a new release later today.

[elbrujohalcon]

Published: https://hex.pm/packages/worker_pool/4.0.2