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
b1e7dea739