New, exit and close span events are generated while the current
context is set to either `None` or the parent span of the span the
event relates to. This causes spans data to be absent from the JSON
output in the case of the `None`, or causes the span data to reference
the parent's span data. Changing the way the current span is
determined allows the correct span to be identified for these
events. Trying to access the events `.parent()` allows access of the
correct span for the `on_event` actions, while using `.current_span()`
works for normal events.

Ref: tokio-rs#1032

Based on feedback by @hawkw, I've improved the test for tokio-rs#1333 to parse
the json output. This is more specifc for the bug and allows easier
testing of the different span `on_events`.

Ref: tokio-rs#1333 (review)

…n_data

Use the `on_records` test method check all events have the correct
context as described in the PR.