[Python] Fix deadlock caused by ExecutorService::close

[BewareMyPower]

Fixes #11847

Motivation

There's a deadlock that might happen when Python client enables custom logging. From stack traces of #11847, we can see there're 3 threads when the Python program hanged:

1. The thread to call ExecutorServiceProvider::close, which waits until all worker threads of ExecutorService completed by std::thread::join.
2. The thread to use Python object for logging. It stuck at PyGILState_Ensure, which tried to acquire Python GIL. It's called in ClientConnection::handleRead. Since all pending events were cancelled by boost::asio::io_service::stop, these callbacks were completed with boost::asio::error::operation_aborted immediately.
3. The worker thread of ExecutorService. It waited until all callbacks including ClientConnection::handleRead completed.

The root cause might be related to Python's GIL issues. It seems like CPython APIs might not work well in C++ destructors. It might be caused by some lifetime issues. But the direct cause is thread 1 was blocked by joining a worker thread.

Modifications

Detach the worker thread instead of join in ExecutorService::close to avoid potential deadlock. The close method could be called in ClientImpl's destructor, which calls shutdown. It's better to call these blocking methods in C++ destructors even without this issue.

In addition, this PR reduces the log level from error to debug for the boost::asio::error::operation_aborted error code, which means the registered event is cancelled by a close event of the event loop.