Unref the pgServices cache timer to prevent blocking process exit

[jsmnbom]

Description

Fixes #2609

Performance impact

unknown - should be none
possibly positive in case user code anywhere is waiting for process exit?

Security impact

unknown - should be none.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

I have no idea how/if this would be tested - sorry, and it's such a tiny change so dunno about the others.

[changeset-bot]

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Latest commit: d9a4170

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