Design the data layer for a global chat application similar to WhatsApp or Discord. Users are distributed globally, and the system must handle millions of messages per second. Analyze the trade-offs between Strong Consistency, Eventual Consistency, and Causal Consistency for this specific use case. Your response should include: 1) A recommendation on which consistency model to use for message delivery and read receipts. 2) An explanation of how to handle conflicts (e.g., concurrent edits, message reordering) based on your chosen model. 3) A selection of a specific database technology (e.g., Cassandra, DynamoDB, CockroachDB) that supports your architectural requirements. 4) A detailed schema design showing how conversations, messages, and user states are stored and indexed.