Basic async XML codec with a simple test

Motivation:
Integrate asynchronous XML parsing as Netty codec.

Modifications:
Not codec implemented (extracted from other codebase).

io.netty.handler.codec.xml.XmlDecoder decodec XML fed by Netty asynchronously without blocking.

Result:
Asynchronous XML stream parsing.
Here is the PR: https://github.com/FasterXML/aalto-xml/pull/24