Fix inaccurate CurrentServerTime

The `Stopwatch.ElapsedTicks` property is _timer_ ticks, which is not the same as the `DateTime.Ticks` according to the note in the remarks section of [the documentation](https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.stopwatch.elapsedticks?view=netcore-3.1#remarks) (and my testing that led to this discovery).

Try the following simple c# sample to see the difference:
```
var serverTimeAtStart = DateTime.UtcNow;
var startedAt = Stopwatch.StartNew();
Thread.Sleep(5000);
var CurrentServerTime = new DateTime(serverTimeAtStart.Ticks + startedAt.ElapsedTicks, DateTimeKind.Utc);
var CorrectServerTime = new DateTime(serverTimeAtStart.Ticks + startedAt.Elapsed.Ticks, DateTimeKind.Utc);
Debug.WriteLine($"Current: {CurrentServerTime}\r\nCorrect: {CorrectServerTime}\r\nNow: {DateTime.UtcNow}");
```

Author: TheDruidsKeeper

src/ServiceStack.Redis/RedisPubSubServer.cs

@@ -60,7 +60,7 @@ public bool AutoRestart
             set => Interlocked.CompareExchange(ref autoRestart, value ? YES : NO, autoRestart);
         }
 
-        public DateTime CurrentServerTime => new DateTime(serverTimeAtStart.Ticks + startedAt.ElapsedTicks, DateTimeKind.Utc);
+        public DateTime CurrentServerTime => new DateTime(serverTimeAtStart.Ticks + startedAt.Elapsed.Ticks, DateTimeKind.Utc);
 
         public long BgThreadCount => Interlocked.CompareExchange(ref bgThreadCount, 0, 0);
 
@@ -582,4 +582,4 @@ public virtual void Dispose()
             DisposeHeartbeatTimer();
         }
     }
-}
+}