Answer each client in its own thread

The server handled one client at a time, so a client that connected and sent nothing blocked every other client. The request handling is now in handle_client, and the main loop starts one thread for each connection.

The server closes a connection that stays silent for 5 seconds, so its thread ends.

README.md

@@ -101,6 +101,9 @@ failed one. It reads a serial BMS every 5 seconds and retries a failed serial
 read after 1 second. Unavailable devices are omitted from the response. Only the
 first request after startup waits, until each device completes one read attempt.
 
+The server answers each client in its own thread, so a slow client does not
+delay the other clients. It closes a connection that stays silent for 5 seconds.
+
 Each Bluetooth read listens for 10 seconds. If a required report is still
 missing, the read continues until the report arrives, for at most 30 seconds in
 total. A read that reaches this limit returns a partial snapshot. Its