Used ContextKey type from go-ns, to prevent having incompatible types

[DavidSubiros]

What

Use ContextKey type from go-ns, to prevent incompatible types in code that uses this library and other libraries that use the go-ns common ContextKey type. This is important because Context accepts interface{} type, so we would not be able to detect mismatches at compile time.

Although this PR looks like a step back, it is created in order to facilitate the transition from go-ns to dp-net. Once everything uses dp-net (and nothing refers to the go-ns ContextKey type directly), we can move the ContextKey type definition to this library, and everything will be consistent.

How to review

• Make sure code change make sense. I tried to use this in dp-api-router, and checked that context values created by other libraries are correctly read now.
• Unit tests should pass.

Who can review

Anyone.

[DavidSubiros]

Closing - at the end we decided to update all libraries and services that depend on this (https://trello.com/c/SvDbv1J8/4814-safely-migrate-services-that-use-go-ns-context-key-to-dp-net)