Middleware should ideally have a `name` attribute

This middleware does not result in a named object once assigned to the `engine`, making it harder to identify & cleanup listeners when a connection is closed, as I had to somewhat hack here:
https://github.com/MetaMask/metamask-extension/pull/6670/files

Start by tracing how middleware is assigned to the engine and how listeners are registered and cleaned up when a connection closes. Review the linked MetaMask pull request for the reported workaround and identify the middleware entry point involved. Done means the assigned middleware has an identifiable name that supports listener cleanup without that workaround.

Written by the indexing model from the issue text.