federation_sender_instances not getting set when multiple fed senders used

[MTRNord]

The federation_sender_instances from https://github.com/matrix-org/synapse/blob/develop/docs/workers.md#synapseappfederation_sender is missing when using more than 1 sender. This is an invalid config according to the documentation

[spantaleev]

As mentioned in #932, we currently don't support more than 1 federation sender worker (if matrix_synapse_workers_federation_sender_workers_count is more than 1, roles/matrix-synapse/tasks/validate_config.yml will yell at you).

We do this because we don't do the federation_sender_instances thing yet. It has just not been implemented and tested yet.

If someone were to populate matrix_synapse_workers_enabled_list manually with more than 1 federation sender worker (instead of relying on dynamic worker list building via variables like matrix_synapse_workers_federation_sender_workers_count), then we won't catch it during validation and they may supposedly end up with a non-working configuration.