* Update License header for all source files
- Add Apache 2.0 header to source files that did not have one.
- Update all existing headers dated to 2019 to be 2020
- Remove comma from License header to comply with the Apache 2.0
guidelines.
* Update Copyright notice
Use the standard Copyright notices outlined by the
[CNCF](https://github.com/cncf/foundation/blob/master/copyright-notices.md#copyright-notices)
* Remove binary propagators
They are in process of being dropped from the specification and we
haven't be using them anywhere in the project. Can reintroduce them
later.
* Rename Supplier to HTTPSupplier
The supplier is used only in HTTP propagators currently. It's not
clear if it will be useful for binary propagators if they get to be
specified at some point.
* Rework propagation interfaces
The biggest change here is that HTTP extractors return a new context
with whatever information the propagator is able to retrieve from the
supplier. Such interface does not hardcode any extractor's
functionality (like it was before by explicitly returning a span
context and correlation context) and makes it easy to chain multiple
propagators.
Injection part hasn't changed.
* Add Propagators interface
This interface (and its default implementation) is likely going to be
the propagation API used the most. Single injectors, extractors or
propagators are likely going to be used just as parameters to the
Option functions that configure the Propagators implementation.
* Drop noop propagator
It's rather pointless - just create an empty Propagators instance.
* Fix wrong name in docs
Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>
