RFR: 8263903: Use Cleaner instead of finalize to auto stop Timer thread [v3]

[Kim Barrett]

On Tue, 23 Mar 2021 18:36:24 GMT, Brent Christian <bchristi at openjdk.org> wrote:

>> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   make ThreadReaper constructor non-public
>
> Marked as reviewed by bchristi (Reviewer).

Thanks all for reviews.

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PR: https://git.openjdk.java.net/jdk/pull/3106