Refine pfio.cache.HTTPCache

[y1r]

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

• 8ab92df
  • Reports error (i.e. unexpected HTTP status code or exception) to user instead of stop working
• 36b1df8
  • We can expect that the HTTP backend doesn't return many redirect or errors, so we limit the number of retries to one.
• b138d89
  • We can expect that the HTTP backend processes requests faster I/O, so we shorten the timeout.
• 72c9da4
  • 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

[pfn-ci-bot]

Successfully created a job for commit 72c9da4:

• Dashboard for commit 72c9da4

[pfn-ci-bot]

Successfully created a job for commit 72c9da4:

• Dashboard for commit 72c9da4

[y1r]

/test

[pfn-ci-bot]

Successfully created a job for commit 957ea2f:

• Dashboard for commit 957ea2f

[pfn-ci-bot]

Successfully created a job for commit 957ea2f:

• Dashboard for commit 957ea2f

[kuenishi]

@y1r I've fixed the main CI at #295 (just removed flake8), so please rebase and test this pull request again.

[y1r]

/test

[pfn-ci-bot]

Successfully created a job for commit e995632:

• Dashboard for commit e995632

[pfn-ci-bot]

Successfully created a job for commit e995632:

• Dashboard for commit e995632

[y1r]

/test

[pfn-ci-bot]

Successfully created a job for commit 42a8e16:

• Dashboard for commit 42a8e16

[pfn-ci-bot]

Successfully created a job for commit 42a8e16:

• Dashboard for commit 42a8e16