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.
