Made some Buffer subclasses final

src/main/java/net/schmizz/sshj/common/Buffer.java

@@ -51,7 +51,7 @@ public class Buffer<T extends Buffer<T>> {
         }
     }
 
-    public static class PlainBuffer
+    public static final class PlainBuffer
             extends Buffer<PlainBuffer> {
 
         public PlainBuffer() {

src/main/java/net/schmizz/sshj/common/SSHPacket.java

@@ -37,7 +37,7 @@ package net.schmizz.sshj.common;
 
 import java.util.Arrays;
 
-public class SSHPacket
+public final class SSHPacket
         extends Buffer<SSHPacket> {
 
     public SSHPacket() {

src/main/java/net/schmizz/sshj/sftp/Request.java

@@ -17,7 +17,7 @@ package net.schmizz.sshj.sftp;
 
 import net.schmizz.concurrent.Promise;
 
-public class Request
+public final class Request
         extends SFTPPacket<Request> {
 
     private final PacketType type;

src/main/java/net/schmizz/sshj/sftp/Response.java

@@ -17,7 +17,7 @@ package net.schmizz.sshj.sftp;
 
 import net.schmizz.sshj.common.Buffer;
 
-public class Response
+public final class Response
         extends SFTPPacket<Response> {
 
     public static enum StatusCode {