AsyncSocket doesn't support movable buffer

Maybe I am missing something. According to this description from AsyncTransport.h:
```
/**
* When data becomes available, isBufferMovable() will be invoked to figure
* out which API will be used, readBufferAvailable() or
* readDataAvailable(). If isBufferMovable() returns true, that means
* ReadCallback supports the IOBuf ownership transfer and
* readBufferAvailable() will be used. Otherwise, not.
* By default, isBufferMovable() always return false. If
* readBufferAvailable() is implemented and to be invoked, You should
* overwrite isBufferMovable() and return true in the inherited class.
*
* This method allows the AsyncSocket/AsyncSSLSocket do buffer allocation by
* itself until data becomes available. Compared with the pre/post buffer
* allocation in getReadBuffer()/readDataAvailabe(), readBufferAvailable()
* has two advantages. First, this can avoid memcpy. E.g., in
* AsyncSSLSocket, the decrypted data was copied from the openssl internal
* buffer to the readbuf buffer. With the buffer ownership transfer, the
* internal buffer can be directly "moved" to ReadCallback. Second, the
* memory allocation can be more precise. The reason is
* AsyncSocket/AsyncSSLSocket can allocate the memory of precise size
* because they have more context about the available data than
* ReadCallback. Think about the getReadBuffer() pre-allocate 4072 bytes
* buffer, but the available data is always 16KB (max OpenSSL record size).
*/

virtual bool isBufferMovable() noexcept {
return false;
}
```

But even I return true in my ReadCallback object, it never call readBufferAvailable(). Search for the code, it seems that only AsyncPipe supports that. Is that right? I am using v2020.09.07. I also checked the latest version of AsyncSocket, it doesn't call isBufferMovalble() ever.