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/valyala/bytebufferpool/doc.go

@@ -0,0 +1,7 @@
+// Package bytebufferpool implements a pool of byte buffers
+// with anti-fragmentation protection.
+//
+// The pool may waste limited amount of memory due to fragmentation.
+// This amount equals to the maximum total size of the byte buffers
+// in concurrent use.
+package bytebufferpool