[Fix][Plugin] sw_flask general exceptions handled (#93)
* sw_flask fix will handle errors like returning the wrong type from a handler or other internal errors.
* Updated StackedSpan to track depth, and made depth variable instance instead of class level (this was a bug).
* Changed how SpanContext decides when all spans finished to write Segment data, now counts span start / stops which should work better across different async scenarios.
* Changed new_exit_span() with span.inject() to work simpler like the NodeJS agent, now plugins inject directly themselves if they need to.
* Removed carrier from plugins which didn't actually use it.
12 files changed
