Fix for race condition in NuGetExeDownloaderService

Same as PR https://github.com/NuGet/NuGetGallery/pull/831 - Just targeting the 'dev' branch since we wanted to get this in to 'preview' in time for testing.

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Fixes #830. After talking to @pranavkm, we decided that a lock isn't necessary here, which enables us to use await and make everything sunshine and rainbows.