v0.15.4

• (client) Deprecated MSC2716 methods and added new Beeper-specific batch send methods, as upstream MSC2716 support has been abandoned.
• (client) Added proper error handling and automatic retries to media downloads.
• (crypto, bridge) Added option to remove all keys that were received before the automatic ratcheting was implemented (in v0.15.1).
• (dbutil) Added JSON utility for writing/reading arbitrary JSON objects to the db conveniently without manually de/serializing.