unbundle20: allow generic dispatch between unbundlers
We now take full advantage of the 'getunbundler' function by using a
'{version -> unbundler-class}' mapping. This map currently contains a single
entry but will make it easy to support more versions from an extension/the
future.
At some point, this map will probably contain bundler-class information too,
in the same fashion the packer map does. However, this is not critically required
right now so it will happen by itself when needed.
The main target is to allow HG2Y support in an extension to ease transition of
companies using the experimental protocol in production (yeah...) But I've no
doubt this will be useful when playing with a future HG21.