[statsd] Restore buffering state when exiting context manager

[sgnn7]

What does this PR do?

When exiting the context or when manual close_buffer is invoked, we
should restore the buffering state that existed before the
context/manual buffer changes were applied.

Fixes: #714

Description of the Change

Buffering state is restored when buffer is manually closed or the managed context is left.

Alternate Designs

N/A

Possible Drawbacks

N/A

Verification Process

Unit tests cover the functionality but manual test can be done by following directions in the upstream issue

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.

[sgnn7]

/azp run

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