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/workers/util/inject_systemd_services_for_worker.yml

@@ -0,0 +1,6 @@
+
+- set_fact:
+    matrix_synapse_worker_systemd_service_name: "matrix-synapse-worker-{{ matrix_synapse_worker_details.type }}-{{ matrix_synapse_worker_details.port }}.service"
+
+- set_fact:
+    matrix_systemd_services_list: "{{ matrix_systemd_services_list + [matrix_synapse_worker_systemd_service_name] }}"

roles/matrix-synapse/tasks/workers/util/setup_files_for_worker.yml

@@ -0,0 +1,20 @@
+
+- set_fact:
+    matrix_synapse_worker_systemd_service_name: "matrix-synapse-worker-{{ matrix_synapse_worker_details.type }}-{{ matrix_synapse_worker_details.port }}"
+
+- set_fact:
+    matrix_synapse_worker_container_name: "{{ matrix_synapse_worker_systemd_service_name }}"
+
+- set_fact:
+    matrix_synapse_worker_config_file_name: "worker.{{ matrix_synapse_worker_details.type }}_{{ matrix_synapse_worker_details.port }}.yaml"
+
+- name: Ensure configuration exists for {{ matrix_synapse_worker_systemd_service_name }}
+  template:
+    src: "{{ role_path }}/templates/synapse/worker.yaml.j2"
+    dest: "{{ matrix_synapse_config_dir_path }}/{{ matrix_synapse_worker_config_file_name }}"
+
+- name: Ensure systemd service exists for {{ matrix_synapse_worker_systemd_service_name }}
+  template:
+    src: "{{ role_path }}/templates/synapse/systemd/matrix-synapse-worker.service.j2"
+    dest: "{{ matrix_systemd_path }}/{{ matrix_synapse_worker_systemd_service_name }}.service"
+    mode: 0644