[#2623] Release local references to guard against StackOverflow in JNI

Motivation:

When we do a (env*)->GetObjectArrayElement(...) call we may created many local references which will only be cleaned up once we exist the native method. Thus a lot of memory can be used and so a StackOverFlow may be triggered. Beside this the JNI specification only say that an implementation must cope with 16 local references.

Modification:

Call (env*)->ReleaseLocalRef(...) to release the resource once not needed anymore.

Result:

Less memory usage and guard against StackOverflow

transport-native-epoll/src/main/c/io_netty_channel_epoll_Native.c

@@ -692,6 +692,12 @@ JNIEXPORT jlong JNICALL Java_io_netty_channel_epoll_Native_writev(JNIEnv * env,
         iov[iovidx].iov_base = buffer + pos;
         iov[iovidx].iov_len = (size_t) (limit - pos);
         iovidx++;
+
+        // Explicit delete local reference as otherwise the local references will only be released once the native method returns.
+        // Also there may be a lot of these and JNI specification only specify that 16 must be able to be created.
+        //
+        // See https://github.com/netty/netty/issues/2623
+        (*env)->DeleteLocalRef(env, bufObj);
     }
 
     ssize_t res;
@@ -728,6 +734,12 @@ JNIEXPORT jlong JNICALL Java_io_netty_channel_epoll_Native_writev(JNIEnv * env,
             incrementPosition(env, bufObj, len);
             written -= len;
         }
+
+        // Explicit delete local reference as otherwise the local references will only be released once the native method returns.
+        // Also there may be a lot of these and JNI specification only specify that 16 must be able to be created.
+        //
+        // See https://github.com/netty/netty/issues/2623
+        (*env)->DeleteLocalRef(env, bufObj);
     }
     return res;
 }