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/labstack/echo/v4/go.mod

@@ -0,0 +1,11 @@
+module github.com/labstack/echo/v4
+
+go 1.12
+
+require (
+	github.com/dgrijalva/jwt-go v3.2.0+incompatible
+	github.com/labstack/gommon v0.3.0
+	github.com/stretchr/testify v1.4.0
+	github.com/valyala/fasttemplate v1.0.1
+	golang.org/x/crypto v0.0.0-20190701094942-4def268fd1a4
+)