Formalize typedvalues implementation

[erwinvaneyk]

This change fixes long overdue issues with the typedvalues implementation. Due to initial prototyping and initial lack of clear context boundaries, this implementation has been fragile and the source of issues. Also, it's convoluted implementation made it difficult for external contributors to understand.

This PR solves this by redesigning the typedvalues package to be: (1) simple/low complexity (2) make the API less cryptic/more self-explanatory, and (3) more structured when it comes to HTTP parsing and formatting. Towards this end, this change...

• switches the serialization underlying the typedvalues to use protobuf only, rather than a per-type difference.
• centralizes all typedvalues-specific logic in the typedvalues package. Moving the protobuf declarations from a types package
• Renamed labels on the typedvalues struct to metadata, to better illustrate its intended use.
• Renamed parse/format of typedvalues to wrap/unwrap to better indicate the behaviour.
• Extracted the parsers and formatters for HTTP requests to explicit interfaces in the httpconv package
• Extracted all control logic related typedvalues to the controlflow package, in part to avoid a cyclic dependency between types and typedvalues.

Given that this formalization uncovers a lot of unhandled edge cases in the initial fragile implementation, several things were changed besides the typedvalues:

• Updated the dependencies on kubernetes and Golang in Travis CI.
• Added a check for the common error of using gogo-protobuf implementation instead of the golang-protobuf implementation.
• Added explicit dependencies on testify and mergo.
• Moved the environment proxy to a subpackage of the apiserver, instead of in the fnenv.
• Renamed and restructured the apiserver api.