Improve shutdown logic: Wait until no requests are made

[motoki317]

What this PR does / why we need it:

Hello!
This PR mainly introduces 3 changes:

• Improves shutdown logic of the controller.
  • The controller now waits until no more requests are seen to the nginx process before exiting. This is done by polling /nginx_status (stub_status) page, and see if the requests handled number goes up.
  • resolves Zero downtime upgrade #6928.
• Changes the grace-shutdown-period in chart values to a more reasonable default.
  • Kubernetes pods are terminated (sent a SIGTERM) and removed from Service endpoints in parallel, so pods are expected to wait a bit before stopping to accept new connections from the upstream - this is the very reason this PR exists. It is generally a good idea in Kubernetes to wait a little bit after receiving a SIGTERM signal before exiting - I have changed the default wait time from 0s to 10s.
  • resolves Zero downtime upgrade #6928 along with the above change.
• Removes the unnecessary (verbose) wait-shutdown binary and preStop hook.
  • This preStop hook seemed to serve no purpose - see Distinguish wait-shutdown command from standard k8s SIGTERM #6287 for discussion.
  • resolves Distinguish wait-shutdown command from standard k8s SIGTERM #6287.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
  • I should note that: this PR technically introduces a breaking change to the default configuration values, but shouldn't be much of a problem to the users.
• Documentation only

Which issue/s this PR fixes

fixes #6928
fixes #6287

How Has This Been Tested?

• I have built docker image on this branch and manually tested the improved shutdown logic feature.
  • See https://gist.github.com/motoki317/902637f6761e289098925d7db1999f43 for how I tested it
• I have added one E2E test spec to verify that the improved shutdown logic indeed works. I have run all E2E tests locally and verified that all tests pass.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: motoki317 / name: motoki317 (073786e, 404e72d, 55ae9c0, 30cc850, 0b903e9, d740e5f, 60f8c04)

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