feat(synapse): allow using multiple federation workers

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -2859,7 +2859,9 @@ opentracing:
 # Disables sending of outbound federation transactions on the main process.
 # Uncomment if using a federation sender worker.
 #
-#send_federation: false
+{% if matrix_synapse_workers_federation_sender_workers_count|int > 0 %}
+send_federation: false
+{% endif %}
 
 # It is possible to run multiple federation sender workers, in which case the
 # work is balanced across them.
@@ -2869,8 +2871,13 @@ opentracing:
 # started, to ensure that all instances are running with the same config (otherwise
 # events may be dropped).
 #
-#federation_sender_instances:
-#  - federation_sender1
+{% if matrix_synapse_workers_federation_sender_workers_count != 0%}
+federation_sender_instances:
+{% for i in range(0, matrix_synapse_workers_federation_sender_workers_count|int)|list %}
+#  -dd federation_sender1
+  - federation_sender:{{ i | int }}
+{% endfor %}
+{% endif %}
 
 # When using workers this should be a map from `worker_name` to the
 # HTTP replication listener of the worker, if configured.