SPIGOT-3351: Replace bundled netty with 4.1.12.Final

pom.xml

@@ -57,6 +57,13 @@
             <version>5.1.42</version>
             <scope>compile</scope>
         </dependency>
+        <!-- bugfixes (netty#6607) -->
+        <dependency>
+            <groupId>io.netty</groupId>
+            <artifactId>netty-all</artifactId>
+            <version>4.1.12.Final</version>
+            <scope>compile</scope>
+        </dependency>
         <!-- testing -->
         <dependency>
             <groupId>junit</groupId>