Prevent DatabaseServerMessenger handle leak

[nzdev]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes

Description

Dispose ManualResetEvent as it is IDisposable.

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[ronaldbarendse]

Hi Chad, thanks for this PR and adhering to the reference Dispose() implementation 👍🏻

Most instances will automatically be disposed of by the MS DI container, but these instances seem to be manually constructed, so we should indeed ensure to property dispose of them, good catch!

[nul800sebastiaan]

Looking good now, thanks @nzdev!