Fix memory leak issue of reading small entries (apache#3844)

Author: codelipenghui

bookkeeper-server/src/main/java/org/apache/bookkeeper/proto/BookieProtoEncoding.java

@@ -271,6 +271,7 @@ public Object encode(Object msg, ByteBufAllocator allocator)
 
                     if (isSmallEntry) {
                         buf.writeBytes(rr.getData());
+                        rr.release();
                         return buf;
                     } else {
                         return ByteBufList.get(buf, rr.getData());