RFR: 8254758: Change G1ServiceThread to be task based [v2]

[Stefan Johansson]

On Tue, 20 Oct 2020 07:34:39 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Albert review
>
> Thanks for your comments Ivan.

Latest commit includes some additional renaming apart from what's been discussed here. Had a chat with Ivan and Albert
to come up with a bit better names and also moved the logic to decide if a task should be rescheduled to a separate
function on the task, rather than using a special value for the `delay_ms()` (former `timeout_ms()`).

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PR: https://git.openjdk.java.net/jdk/pull/734