Change the calculation of parent spans (#9)

* Change the calculation of parent spans

Add a "entered" stack to keep track of enter span/exit span in the
channel read thread rather than the sender.

This allows parent spans to be calculated based on the items in the
stack rather than solely based on enter/exit states.

* Automatically push trace parent span

Now it is no longer necessary to decide on the parent; it will
automatically be pushed when a new trace is started (first span where
span collection is empty for the given trace ID).  This parent wil be
re-calculated if all events are sent on a trace (because the current
span collection for that trace will be consumed and further create
events will create a whole new span collection, including a new parent.

Now, draining a span collection consumes it, so it can no longer be used
once the "send_trace" event is received for a trace.