Allow running postbacks in parallel

[exyi]

I'd expect that postbacks in different queues can run in parallel, but it's not the case, since we drop every commit that's not from the last postback.

This line makes it not work:

https://github.com/riganti/dotvvm/blob/a78b8b0f023ae05cb5373be386b3bd9f702c1174/src/DotVVM.Framework/Resources/Scripts/DotVVM.ts#L188

But there will be most likely more bugs related to that, so we'll leave this bug fix as a feature for 3.0 :)