This is to support census-instrumentation/opencensus-specs#247.

1. Provide a generic context, which is maintained collaboratively by the Python runtime and the OpenCensus library.
2. Provide asyncio/async/await support.
3. Refactor existing context (span_context, trace/execution_context, tags/execution_context, correlation_context).
4. Provide callback API for span start/end.

@c24t @bogdandrutu @discostu105