[concurrentbatchprocessor] Consistent span creation, latency, bi-directional links

[jmacd]

Three minor changes in CBP instrumentation:

• unconditionally create a span, even in cases where there is a single parent; less confusing to always have a span w/ observable latency, otherwise users have to look at each receiver individually.
• end the span after the export call, vs immediately
• link in both directions, allows the incoming requests to link to their outgoing batch easily w/o depending on trace backend features.
  I also ran into some test debts, fixed to cover the new behavior.