RFR: 6542: Improve calls to render in the flame view

[Miroslav Wengner]

On Tue, 9 Jun 2020 21:57:13 GMT, Miroslav Wengner <mwengner at openjdk.org> wrote:

>> Sadly I can still make it go out of whack with these changes:
>> ![image](https://user-images.githubusercontent.com/16906077/83888656-71fc4c80-a74a-11ea-9c03-1cfa0c28569e.png)
>
> I've add a fix.

maybe we can increase the Executor treads in FlameView, I've observed improvements. But calculating the model is still
expensive.

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PR: https://git.openjdk.java.net/jmc/pull/78