Testing parenting handling revealed several issues:
1. By calling "trace_span_start" when CLN_TRACEPARENT is set produces a bogus
entry, for which the span_id is overwritten so we never end it.
2. We don't need to close the remote parent when we close the first child: in
fact, this causes the remaining traces to be detached from the parent!
3. Suspension should return current to the parent, not to NULL.
Now the traces balance as we expect.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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