Run Synapse workers in their own containers

This switches the `docker exec` method of spawning
Synapse workers inside the `matrix-synapse` container with
dedicated containers for each worker.

We also have dedicated systemd services for each worker,
so this are now:
- more consistent with everything else (we don't use systemd
instantiated services anywhere)
- we don't need the "parse systemd instance name into worker name +
port" part
- we don't need to keep track of PIDs manually
- we don't need jq (less depenendencies)
- workers dying would be restarted by systemd correctly, like any other
service
- `docker ps` shows each worker separately and we can observe resource
usage

roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2

@@ -14,7 +14,7 @@
 		ip_hash;
 
 		{% for worker in generic_workers %}
-		server "matrix-synapse:{{ worker.port }}";
+		server "matrix-synapse-worker-{{ worker.type }}-{{ worker.port }}:{{ worker.port }}";
 		{% endfor %}
 	}
 	{% endif %}
@@ -22,7 +22,7 @@
 	{% if frontend_proxy_workers %}
 	upstream frontend_proxy_upstream {
 		{% for worker in frontend_proxy_workers %}
-		server "matrix-synapse:{{ worker.port }}";
+		server "matrix-synapse-worker-{{ worker.type }}-{{ worker.port }}:{{ worker.port }}";
 		{% endfor %}
 	}
 	{% endif %}
@@ -30,7 +30,7 @@
 	{% if media_repository_workers %}
 	upstream media_repository_upstream {
 		{% for worker in media_repository_workers %}
-		server "matrix-synapse:{{ worker.port }}";
+		server "matrix-synapse-worker-{{ worker.type }}-{{ worker.port }}:{{ worker.port }}";
 		{% endfor %}
 	}
 	{% endif %}
@@ -38,7 +38,7 @@
 	{% if user_dir_workers %}
 	upstream user_dir_upstream {
 		{% for worker in user_dir_workers %}
-		server "matrix-synapse:{{ worker.port }}";
+		server "matrix-synapse-worker-{{ worker.type }}-{{ worker.port }}:{{ worker.port }}";
 		{% endfor %}
 	}
 	{% endif %}