Integrate non-blocking XML parser as Netty codec (#2806)

Integrate non-blocking XML parser as Netty codec (#2806)

Modifications:
New codec implemented/extracted.

io.netty.handler.codec.xml.XmlDecoder decodes XML fed by Netty without blocking.

Result:
Non-blocking XML stream parsing.
No use full qualified name. Just use `{@link XmlElement}`.