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/.travis.yml

@@ -0,0 +1,17 @@
+language: go
+go:
+  - 1.11.x
+  - 1.12.x
+  - tip
+env:
+  - GO111MODULE=on
+install:
+  - go get -v golang.org/x/lint/golint
+script:
+  - golint -set_exit_status ./...
+  - go test -race -coverprofile=coverage.txt -covermode=atomic ./...
+after_success:
+  - bash <(curl -s https://codecov.io/bash)
+matrix:
+  allow_failures:
+    - go: tip