Fix this-escape warnings

src/main/java/uk/co/notnull/proxyqueues/ProxyDiscordHandler.java

@@ -40,7 +40,7 @@
 import java.util.Optional;
 import java.util.UUID;
 
-public class ProxyDiscordHandler {
+public final class ProxyDiscordHandler {
 	private final ProxyQueuesImpl plugin;
 	private static VerificationManager verificationManager;
 

src/main/java/uk/co/notnull/proxyqueues/queues/ProxyQueueImpl.java

@@ -55,7 +55,7 @@
 import java.util.concurrent.atomic.AtomicInteger;
 
 
-public class ProxyQueueImpl implements uk.co.notnull.proxyqueues.api.queues.ProxyQueue {
+public final class ProxyQueueImpl implements uk.co.notnull.proxyqueues.api.queues.ProxyQueue {
 
     private final ProxyQueuesImpl proxyQueues;
 

src/main/java/uk/co/notnull/proxyqueues/queues/QueueHandlerImpl.java

@@ -40,7 +40,7 @@
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 
-public class QueueHandlerImpl implements QueueHandler {
+public final class QueueHandlerImpl implements QueueHandler {
 
     private final ConcurrentHashMap<RegisteredServer, ProxyQueue> queues;
     private final SettingsManager settingsManager;

src/main/java/uk/co/notnull/proxyqueues/queues/QueuePlayerImpl.java

@@ -34,7 +34,7 @@
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
 
-public class QueuePlayerImpl implements QueuePlayer {
+public final class QueuePlayerImpl implements QueuePlayer {
 
     private final BossBar bossBar;
     private boolean connecting;