Add ability to shard the federation sender (#7798)

changelog.d/7798.feature

docs/sample_config.yaml

synapse/app/generic_worker.py

synapse/config/federation.py

synapse/config/homeserver.py

synapse/config/server.py

synapse/federation/send_queue.py

synapse/federation/sender/__init__.py

synapse/federation/sender/per_destination_queue.py

synapse/replication/tcp/commands.py

synapse/replication/tcp/handler.py

...s/main/schema/delta/58/10federation_pos_instance_name.sql

synapse/storage/data_stores/main/stream.py

tests/replication/test_federation_ack.py

tests/replication/test_federation_sender_shard.py