reset worker on exception

Author: mythz

src/ServiceStack.Redis/RedisSentinel.cs

@@ -321,6 +321,7 @@ private RedisSentinelWorker GetValidSentinelWorker()
                         OnWorkerError(ex);
 
                     lastEx = ex;
+                    this.worker = null;
                     this.failures++;
                     Interlocked.Increment(ref RedisState.TotalFailedSentinelWorkers);
                 }