The context can be passed from the client applications as a hash.

Every time a new context is added, it will inherit the values of the
previous context. Those previous values will be overridden by any new
ones passed.

A context always has a correlation id that is not empty. If an empty
correlation id is passed, a new one will be generated.

There is a `Labkit::Middleware::Rack` that will set the correlation id
to the current request id. Because of this, every newly started
context within a rack request will get the same correlation id.

Using the `Labkit::Middleware::Sidekiq::Client` middleware the context
active at the moment of scheduling a job will be serialized into the
job.

The `Labkit::Middleware::Sidekiq::Server` middleware will load the
context that was stored on the job, so newly scheduled jobs would
start with the same context. This context could be modified by the
job.

In this iteration the following extra values can be specified on the
context:
- u...