[jfx15] RFR: 8248381: Create a daemon thread for MonocleTimer [v2]

[Kevin Rushforth]

On Sun, 28 Jun 2020 02:28:35 GMT, John Neffenger <github.com+1413266+jgneff at openjdk.org> wrote:

>> Fixes [JDK-8248381](https://bugs.openjdk.java.net/browse/JDK-8248381).
>
> John Neffenger has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove POOL_SIZE constant

Looks good to me. I verified that it does, in fact, fix the regression. As long as @johanvos also approves, then this
is fine to go in for `jfx15`

I think we can address any issues relating to Timer start / stop / pause / resume in a subsequent release. There was
some work done to support pause / resume as part of [JDK-8189926](https://bugs.openjdk.java.net/browse/JDK-8189926),
but it was only implemented on macOS (with follow-up issues filed to implement it on Windows and Linux). It didn't
address any issues with start / stop. In particular we have never supported restarting the JavaFX runtime, so unless
there are other uses of stop besides shutting down the runtime, I don't know that it matters if the timer can be
restarted.

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Marked as reviewed by kcr (Lead).

PR: https://git.openjdk.java.net/jfx/pull/256