/** Prepares for the execution of a task. Returns the prepared
     *  execution context. The recommended implementation of
     *  `prepare` is to return `this`.
     *
     *  This method should no longer be overridden or called. It was
     *  originally expected that `prepare` would be called by
     *  all libraries that consume ExecutionContexts, in order to
     *  capture thread local context. However, this usage has proven
     *  difficult to implement in practice and instead it is
     *  now better to avoid using `prepare` entirely.
     *
     *  Instead, if an `ExecutionContext` needs to capture thread
     *  local context, it should capture that context when it is
     *  constructed, so that it doesn't need any additional
     *  preparation later.
     */