Merge account data streams (#14826)

changelog.d/14826.misc

docs/upgrade.md

synapse/api/constants.py

synapse/handlers/account_data.py

synapse/handlers/initial_sync.py

synapse/handlers/sync.py

synapse/replication/tcp/client.py

synapse/replication/tcp/handler.py

synapse/replication/tcp/streams/__init__.py

synapse/replication/tcp/streams/_base.py

synapse/storage/databases/main/account_data.py

synapse/storage/databases/main/tags.py