We've decided that automatically created spans should be a child of the root transaction.
It'll be a flat list of spans but the timeline is still correct due to the timestamps.

On Cocoa, getsentry/sentry-cocoa#1215 was already closed to keep creating automatic spans from the root.

This is only necessary for Mobile integrations since they have the concept of singleHub, backend instrumentation does not require that since each thread has its own hub/scope.