Stop RUM event publishing on unload instead of beforeUnload

[sukhpreet96]

Motivation

To fix the issues with beforeUnload event.

Changes

Changed stopping of RUM events from beforeUnload to unload

Testing

• Local
• Staging
• Unit
• End to end

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I have gone over the contributing documentation.

[BenoitZugmeyer]

While we are aware of beforeunload shortcomings, we don't intend to change the current strategy at this point, as it would impact reliability. It is still on our mind though, and we might do something about it in the future.