Use Sentry.with_child_span in redis and net/http patches

[sl0thentr0py]

Move away from using span.start_child explicitly and use the top level Sentry.with_child_span.

This is necessary going forward for having some top-level api control for disabling sentry instrumentation if we're using open telemetry.

Note: I had to remove one net/http test for the unsampled case since that behaviour changed now. But this is fine as in principle the sampled flag just controls the final hub.capture_event and not the actual span recording.

[st0012]

This may be nitpicking, but if there's a behaviour change, is it still just a refactoring?
Should the user notice anything from the changes? e.g. different span nesting

[sl0thentr0py]

@st0012 yea fair point, the nesting might change (be more accurate).
I'll move the changelog entry.