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-synapse/tasks/init.yml

@@ -2,6 +2,13 @@
     matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-synapse.service'] }}"
   when: matrix_synapse_enabled|bool
 
+- name: Ensure systemd services for workers are injected
+  include_tasks: "{{ role_path }}/tasks/workers/util/inject_systemd_services_for_worker.yml"
+  with_items: "{{ matrix_synapse_workers_enabled_list }}"
+  loop_control:
+    loop_var: matrix_synapse_worker_details
+  when: matrix_synapse_enabled|bool and matrix_synapse_workers_enabled|bool
+
 - set_fact:
     matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-goofys.service'] }}"
   when: matrix_s3_media_store_enabled|bool