(refac) Zookeeper for service-discovery, kill service-client lib (WIP)

To avoid having to either hard-code or manually configure service addresses (possibly several dozen), and to reduce the project's dependency on docker to deal with routing and discovery, the option to use [Zookeeper](https://zookeeper.apache.org/) to manage services and discovery has been added.

A service registry interface was added, with a Zookeeper implementation and a basic implementation that only works on docker and hard-codes everything.

The last remaining REST service, the assistant-service, has been migrated to gRPC.

This also proved a good time to clear out primordial technical debt from the root of the codebase.  The 'service-client' library has been taken behind the barn and given a last farewell.  It's replaced by a small library for managing gRPC channels.

Since it's no longer used by anything, RxJava has been removed as a dependency from the project.

Although the current state seems reasonably stable, this is a work-in-progress commit.

run/install/docker-compose-barebones-1.yml.template

@@ -103,6 +103,12 @@ services:
       - db:/var/lib/mysql
     networks:
       - wmsa
+  zookeeper:
+    image: zookeeper
+    container_name: "zookeeper"
+    restart: always
+    ports:
+      - "127.0.0.1:2181:2181"
   traefik:
     image: "traefik:v2.10"
     container_name: "traefik"