Migrate the user directory initial population to a background task (#4864)

changelog.d/4864.feature

synapse/handlers/user_directory.py

synapse/storage/background_updates.py

synapse/storage/schema/delta/53/user_dir_populate.sql

synapse/storage/user_directory.py

tests/handlers/test_user_directory.py

tests/storage/test_user_directory.py

tests/unittest.py