Ignore WAKEUP_TASK in SingleThreadEventExecutor.PollTask

[yyjdelete]

Without this, it may lead to an infinite loop.

[yyjdelete]

I didn't find a way to create an test for this. It an concurrency issue and only happen in rare case.

Shutdown will never finished if an task is enqueued during CleanupAndTerminate(). If there is any tasks left, RunAllTasks will return true and call WakeUp(true), and WakeUp(true) will enqueue an WAKEUP_TASK and it makes the next call to RunAllTasks still return true. https://github.com/Azure/DotNetty/blob/a9d0723ba5994c1b98bb43ed452996d51beb1a7c/src/DotNetty.Common/Concurrency/SingleThreadEventExecutor.cs#L115-L119 https://github.com/Azure/DotNetty/blob/a9d0723ba5994c1b98bb43ed452996d51beb1a7c/src/DotNetty.Common/Concurrency/SingleThreadEventExecutor.cs#L375-L381 https://github.com/Azure/DotNetty/blob/a9d0723ba5994c1b98bb43ed452996d51beb1a7c/src/DotNetty.Common/Concurrency/SingleThreadEventExecutor.cs#L317-L328