[Network] Measure real server round-trip times

Time each command container from send to response with QElapsedTimer,
aggregate samples in a fixed-size ring buffer (last/median/p95/max),
and emit aggregated pingStatsUpdated at most once per second so the
hot path stays free of signal traffic. Stats are cleared on
disconnect. Forward the signal through ConnectionController for UI
consumers. Unit-tested in latency_tracker_test.

Took 37 minutes

Took 4 minutes

# Commit time for manual adjustment:
# Took 8 minutes

libcockatrice_protocol/libcockatrice/protocol/pending_command.cpp

@@ -29,3 +29,13 @@ int PendingCommand::tick()
 {
     return ++ticks;
 }
+
+void PendingCommand::startTiming()
+{
+    startTime.start();
+}
+
+qint64 PendingCommand::elapsedMs() const
+{
+    return startTime.isValid() ? startTime.nsecsElapsed() / 1000000 : -1;
+}