Alternative OpenCensus shim implementation proposal

The current OpenCensus shim is implemented here:
https://github.com/open-telemetry/opentelemetry-java/tree/master/opencensus-shim

This implementation has some disadvantages regarding the requirement to use a cache of Spans, spans need to be ended in a certain amount of time, etc. Right now the current approach is to rely on the opencensus-impl to do the work of accumulating events into Spans, export, etc.

First Proposal
The proposal is to make the shim rely on the opentelemetry-sdk to do the heavy work of accumulating events into Spans, export them, etc.

This way you will benefit of the fact that this project is currently very active and maintained, and also I believe this will simplify the implementation, here are some thoughts:

1. The OpenCensus.Tracer will be a simple wrapper of the OpenTelemetry.Tracer with a name "opencensus".
2. The PropagationComponent can be implemented by wrapping the propagators from OpenTelemetry (w3c, b3). One small problem is that you will need to re-implement the BinaryFormat (this is most likely a duplicate code).
3. The clock does not need to be re-implemented. just return the Millis clock.
4. The ExporterComponent can be implemented by wrapping the BatchSpanProcessor, the OpenCensus.SpanExporter.Handler is equivalent with the OpenTelemetry.SpanExporter
5. The TraceConfig is a simple wrapper of the config management in the OpenTelemetry.

Second Proposal
Take a dependency on otel api in the opencensus API and redirect all calls. This may not be possible until GA.

Start by reviewing the existing implementation in opencensus-shim and compare it with the two proposals in the issue. Determine which proposal is viable and what compatibility constraints apply; done requires an agreed design before implementation can be scoped.