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/setup.yml

@@ -1,5 +1,19 @@
 ---
 
+# A previous version of the worker setup used this.
+# This is a temporary cleanup for people who ran that version.
+- name: Ensure old matrix-synapse.service.wants directory is gone
+  file:
+    path: "{{ matrix_systemd_path }}/matrix-synapse.service.wants"
+    state: absent
+
+# Same. This was part of a previous version of the worker setup.
+# No longer necessary.
+- name: Ensure matrix-synapse-worker-write-pid script is removed
+  file:
+    path: "{{ matrix_local_bin_path }}/matrix-synapse-worker-write-pid"
+    state: absent
+
 - include_tasks: "{{ role_path }}/tasks/workers/setup_install.yml"
   when: "matrix_synapse_enabled|bool and matrix_synapse_workers_enabled|bool"