WIP: Context Propagation @ OTEP 66

[jmacd]

This implements a proof of concept for the proposal in OTEP 66.

The new API Propagators API is found in:
api/context/propagation

The new global Propagators is added to api/global.

These new packages are refactored from existing propagators:
api/trace/propagation
api/context/baggage/propagation

This has been prepared to support OTEP 66, as such it is not in ready-to-merge form. Several new tests should be introduced for the new functionality introduced.

[jmacd]

There are still some correctness issues here and the confusion discussed in open-telemetry/oteps#68 is real. We have both a SpanContext and a Span in the context.Context and it's not clear this is what we want. Presently the "current" SpanContext and Span are lacking coordination, so the Tracer.StartSpan implementation is using the current Span's SpanContext which is not always correct. (There are no tests that uncover this bug though.) This remains a work in progress.

[jmacd]

This PR removes the non-compliant ChildOf and FollowsFrom interfaces and the Relation type, which were inherited from OpenTracing via the initial prototype. This exposed the need to tighten up OTEP 66 with regards to clarifying when to use the current Span and when to use an extracted SpanContext as the parent when specifying the explicit and/or implicit parent context. The logic I propose is here, which says to use the remote span context if its TraceID differs from the current Span.

https://github.com/open-telemetry/opentelemetry-go/pull/381/files#diff-0f0866c95e7bc01ff2da6b0ccf5272e2

@mwear and @tedsuo will follow up in the spec.

[jmacd]

FYI this PR is a bit difficult to review because it mixes refactoring with several API changes. To really merge this properly it should be split into several steps.

[jmacd]

FYI the OpenTracing bridge is broken by this PR. It will take more work.