The sync _session_has_compression_in_flight sat on the message hot path
and blocked the event loop twice: under session_store._lock during
_ensure_loaded_locked (JSON read) and via db.get_compression_lock_holder
(SQLite SELECT). Async-ify the method and offload both sources via
asyncio.to_thread; await the call site in _handle_active_session_busy_message.
