* Serverless architecture in this case includes one that utilizes
      Lambda and API Gateway.
    * A new "Serverless" context is created to give the abstraction of
      Segments being the toplevel entities but is then converted to a
      subsegment upon transmission to the data plane.
      These segments are called MimicSegments.  All generated
      segments have a parent segment that is the FacadeSegment.
    * Currently supports Flask and Django as middlewares; this has been
      confirmed to be natively working with Zappa if the application is
      running under Flask/Django.

* Renamed ServerlessContext to ServerlessLambdaContext to be more precise about its purpose.
* Instead of explicitly using Context class's put_subsegment/put_segment methods, use LambdaContext's
grandparent class to store. LambdaContext's methods are really only used to acquire the FacadeSegment.
* Parameters when initializing the mimic segment are now required.
* Unit tests added to test these changes.

* Mimic Segments now perform no-op on segment-only methods.
* Added comments to address this contract.
* Unit tests to ensure segment-only methods don't add data
to the mimic segment. Serialization methods are also tested
to ensure this.

* Revised design; no more serverless context.
* Subsegment generated when the middleware adds request
* Unit tests to ensure that these subsegments are added on top of the facade segment

* Comment change added for save_origin_trace_header