Changes to make Velocity more amenable to memory leak detection

proxy/src/main/java/com/velocitypowered/proxy/Velocity.java

@@ -52,10 +52,9 @@ public class Velocity {
       System.setProperty("io.netty.native.workdir", System.getProperty("velocity.natives-tmpdir"));
     }
 
-    // Disable the resource leak detector by default as it reduces performance. Allow the user to
-    // override this if desired.
+    // Set the ADVANCED level of the leak detector. Time to go hunting.
     if (System.getProperty("io.netty.leakDetection.level") == null) {
-      ResourceLeakDetector.setLevel(Level.DISABLED);
+      ResourceLeakDetector.setLevel(Level.ADVANCED);
     }
   }
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftVarintFrameDecoder.java

@@ -59,7 +59,7 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
       } else {
         int minimumRead = bytesRead + readVarint;
         if (in.isReadable(minimumRead)) {
-          out.add(in.retainedSlice(varintEnd + 1, readVarint));
+          out.add(in.copy(varintEnd + 1, readVarint));
           in.skipBytes(minimumRead);
         }
       }