worker: use openapi spec and generated code

Write an openapi spec for the worker API and use `deepmap/oapi-codegen`
to generate scaffolding for the server-side using the `labstack/echo`
server.

Incidentally, echo by default returns the errors in the same format that
worker API always has:

    { "message": "..." }

The API itself is unchanged to make this change easier to understand. It
will be changed to better suit our needs in future commits.

vendor/github.com/pkg/errors/.travis.yml

@@ -0,0 +1,10 @@
+language: go
+go_import_path: github.com/pkg/errors
+go:
+  - 1.11.x
+  - 1.12.x
+  - 1.13.x
+  - tip
+
+script:
+  - make check