CSHARP-3429: Setting DNS thread as background

Author: BorisDog

src/MongoDB.Driver.Core/Core/Clusters/DnsMonitor.cs

@@ -68,7 +68,11 @@ public DnsMonitor(IDnsMonitoringCluster cluster, IDnsResolver dnsResolver, strin
         // public methods
         public Thread Start()
         {
-            var thread = new Thread(ThreadStart);
+            var thread = new Thread(ThreadStart)
+            {
+                IsBackground = true
+            };
+
             thread.Start();
             return thread;
         }

tests/MongoDB.Driver.Core.Tests/Core/Clusters/MultiServerClusterTests.cs

@@ -18,18 +18,18 @@
 using System.Linq;
 using System.Net;
 using System.Threading;
+using System.Threading.Tasks;
 using FluentAssertions;
+using MongoDB.Bson;
 using MongoDB.Bson.TestHelpers;
 using MongoDB.Driver.Core.Configuration;
 using MongoDB.Driver.Core.Events;
+using MongoDB.Driver.Core.Helpers;
 using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Core.Servers;
-using MongoDB.Driver.Core.Helpers;
+using MongoDB.Driver.Core.Tests.Core.Clusters;
 using Moq;
 using Xunit;
-using MongoDB.Bson;
-using MongoDB.Driver.Core.Tests.Core.Clusters;
-using System.Threading.Tasks;
 
 namespace MongoDB.Driver.Core.Clusters
 {
@@ -298,6 +298,7 @@ public void Initialize_should_start_dns_monitor_thread_when_scheme_is_MongoDBPlu
                 subject.Initialize();
 
                 subject._dnsMonitorThread().Should().NotBeNull();
+                subject._dnsMonitorThread().IsBackground.Should().BeTrue();
             }
         }