Implement Correlations API

This ticket is to track implementing the Correlations API to match the spec proposed here: https://github.com/open-telemetry/opentelemetry-specification/pull/420

See [otep 66](https://github.com/open-telemetry/oteps/blob/master/text/0066-separate-context-propagation.md#Correlations-API) for more details.