webhook: enhance certwatcher for atomicity and serialization

[black-dragon74]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This patch includes a set of enhancements to the certwatcher, being:

• Use sync.RWMutex to avoid serialization bottlenecks
• Watch the parent directory instead of the actual files (fsnotify watches inodes).
  • This takes care of the atomic writes by editors such as vi which rename a temp file instead of writing to it.
• Makes ReadCertificate private as it will always be called internally.

Special notes for your reviewer:
NA

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @black-dragon74. Thanks for your PR.

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[black-dragon74]

/ok-to-test

[k8s-ci-robot]

@black-dragon74: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/ok-to-test

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[Rakshith-R]

/ok-to-test

[Rakshith-R]

/ok-to-test

[black-dragon74]

/assign @xing-yang