Refine pfio.cache.HTTPCache

[y1r]

This PR refines HTTPCache to support fault-tolerance and multiprocessing with fork-server execution model.

• https://github.com/pfnet/pfio/commit/8ab92dfde2a0bcac909eb298b6bf732df50bc122
  • Reports error (i.e. unexpected HTTP status code or exception) to user instead of stop working
• https://github.com/pfnet/pfio/commit/36b1df82de8a0fe6f4547580c007d0fbeb4ee9b4
  • We can expect that the HTTP backend doesn't return many redirect or errors, so we limit the number of retries to one.
• https://github.com/pfnet/pfio/commit/b138d89c923259dff13837bc572792e991d6f33c
  • We can expect that the HTTP backend processes requests faster I/O, so we shorten the timeout.
• https://github.com/pfnet/pfio/commit/72c9da444218912ccd652881f0cf2067bb382cfe
  • multiprocessing with fork-server execution model requires picklable parameters. The connection pool doesn't support pickle so we re-create the pool if we detected the fork.

[y1r]

/test

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[y1r]

/test

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[kuenishi]

@y1r I've fixed the main CI at https://github.com/pfnet/pfio/pull/295 (just removed flake8), so please rebase and test this pull request again.

[y1r]

/test

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[y1r]

/test

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