Span structure for messaging scenarios (open-telemetry/oteps#220)

This OTEP aims at defining consistent conventions about what spans to create for messaging scenarios, and at defining how those spans relate to each other. Instrumentors should be able to rely on a consistent set of conventions, as opposed to deducing conventions from a set of examples.

This was split from OTEP #192, which became too comprehensive.

Author: pyohannes