Remove unneeded _sealed_ keywords. Update docs.

Frameworks/MQTTnet.NetStandard/Server/GetOrCreateClientSessionResult.cs

@@ -1,6 +1,6 @@
 namespace MQTTnet.Server
 {
-    public sealed class GetOrCreateClientSessionResult
+    public class GetOrCreateClientSessionResult
     {
         public bool IsExistingSession { get; set; }
 

Frameworks/MQTTnet.NetStandard/Server/MqttClientKeepAliveMonitor.cs

@@ -7,7 +7,7 @@ using MQTTnet.Packets;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttClientKeepAliveMonitor
+    public class MqttClientKeepAliveMonitor
     {
         private readonly Stopwatch _lastPacketReceivedTracker = new Stopwatch();
         private readonly Stopwatch _lastNonKeepAlivePacketReceivedTracker = new Stopwatch();

Frameworks/MQTTnet.NetStandard/Server/MqttClientPendingMessagesQueue.cs

@@ -11,7 +11,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttClientPendingMessagesQueue : IDisposable
+    public class MqttClientPendingMessagesQueue : IDisposable
     {
         private readonly AsyncAutoResetEvent _queueAutoResetEvent = new AsyncAutoResetEvent();
         private readonly IMqttServerOptions _options;

Frameworks/MQTTnet.NetStandard/Server/MqttClientSession.cs

@@ -12,7 +12,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttClientSession : IDisposable
+    public class MqttClientSession : IDisposable
     {
         private readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
 

Frameworks/MQTTnet.NetStandard/Server/MqttClientSessionsManager.cs

@@ -12,7 +12,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttClientSessionsManager : IDisposable
+    public class MqttClientSessionsManager : IDisposable
     {
         private readonly ConcurrentDictionary<string, MqttClientSession> _sessions = new ConcurrentDictionary<string, MqttClientSession>();
         private readonly AsyncLock _sessionPreparationLock = new AsyncLock();

Frameworks/MQTTnet.NetStandard/Server/MqttClientSubscriptionsManager.cs

@@ -7,7 +7,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttClientSubscriptionsManager
+    public class MqttClientSubscriptionsManager
     {
         private readonly ConcurrentDictionary<string, MqttQualityOfServiceLevel> _subscriptions = new ConcurrentDictionary<string, MqttQualityOfServiceLevel>();
         private readonly IMqttServerOptions _options;

Frameworks/MQTTnet.NetStandard/Server/MqttRetainedMessagesManager.cs

@@ -7,7 +7,7 @@ using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttRetainedMessagesManager
+    public class MqttRetainedMessagesManager
     {
         private readonly ConcurrentDictionary<string, MqttApplicationMessage> _messages = new ConcurrentDictionary<string, MqttApplicationMessage>();
         private readonly IMqttNetChildLogger _logger;

Frameworks/MQTTnet.NetStandard/Server/MqttServerDefaultEndpointOptions.cs

@@ -2,7 +2,7 @@
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttServerDefaultEndpointOptions
+    public class MqttServerDefaultEndpointOptions
     {
         public bool IsEnabled { get; set; } = true;
 

Frameworks/MQTTnet.NetStandard/Server/MqttServerTlsEndpointOptions.cs

@@ -2,7 +2,7 @@
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttServerTlsEndpointOptions
+    public class MqttServerTlsEndpointOptions
     {
         public bool IsEnabled { get; set; }