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/mattn/go-isatty/go.mod

@@ -0,0 +1,5 @@
+module github.com/mattn/go-isatty
+
+require golang.org/x/sys v0.0.0-20191008105621-543471e840be
+
+go 1.14