fix(core): use separate buffer for span close events

[CrabNejonas]

This changes the span events buffering to put close events into a separate buffer. This is important because close events are used for animations and calculations in the frontend, completely missing out on them leads to incorrectness.

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[johann-crabnebula]

I tested this and this does improve droppage of close events. But I feel like we should not only focus on them.