Use a chain cover index to efficiently calculate auth chain difference (#8868)

changelog.d/8868.misc

docs/auth_chain_diff.dot

docs/auth_chain_diff.dot.png

docs/auth_chain_difference_algorithm.md

synapse/storage/database.py

synapse/storage/databases/main/event_federation.py

synapse/storage/databases/main/events.py

synapse/storage/databases/main/room.py

...e/databases/main/schema/delta/59/04_event_auth_chains.sql

...es/main/schema/delta/59/04_event_auth_chains.sql.postgres

synapse/util/iterutils.py

tests/storage/test_event_chain.py

tests/storage/test_event_federation.py

tests/util/test_itertools.py