Store ScopeShim in context

This uses the OpenTelemetry context management mechanism to store a
ScopeShim object in order to make active return the same object as the
one returned by start_active_span.

WIP: there is one failing test case, apparently the context does not get
cleared before running every test case. This seems related to the order
in which test cases are being run.

ext/opentelemetry-ext-opentracing-shim/tests/test_shim.py

@@ -532,6 +532,8 @@ def test_extract_empty_context_returns_invalid_context(self):
         try:
             carrier = {}
 
+            from ipdb import set_trace
+            set_trace()
             ctx = self.shim.extract(opentracing.Format.HTTP_HEADERS, carrier)
             self.assertEqual(ctx.unwrap(), trace.INVALID_SPAN_CONTEXT)
         finally: