Fix a deadlock in the log-stream handler that caused all logs
requests to hang: Connection used a single Mutex<JsonFramed> for
both reads and writes, so the serve loop holding the read lock
blocked the spawned notification task from writing. Split
Connection into separate reader and writer mutexes.
Also fix a response/notification ordering race: the log task now
waits for an explicit ready signal sent by serve after writing the
LOGS response, ensuring notifications never arrive at the client
before their initiating response.
logaritmisk