Provide parent span for duration of transaction

[matthughes]

Skunk emits a span for allocation of a session and freeing of a session but it would be nice to have a span that marks the duration of a session to track long running transactions. Some tracing tools might be able to extrapolate by subtracting pool.allocate from pool.free but Jaeger cannot.

I think something like this would work:

pool.allocate -> pool.use -> pool.free