Implement corresponding service-side functionality to support TransportClientMultiplexer

[robertbartel]

Several new types - e.g., TransportClientMultiplexer - have been developed in #417 to address problems with sharing a single connection and message ordering. However, at this time, those changes only fully address these problems on the "client" side of connections.

Corresponding behavior needs to be implemented on the service side, through service manager classes directly and/or via AbstractRequestHandler and its subclasses.

This issue blocks the completion of #417.

[christophertubbs]

Don't know if this helps at all, but I addressed a similar issue with the evaluation service connections by attaching optional request_ids to websocket requests and responses. 50k messages might be flying back and forth, but the client would know to update element x instead of element y because it was able to acknowledge the correct accompanying id.