changegroup: use compression engines API The new API doesn't have the equivalence for None and 'UN' so we introduce code to use 'UN' explicitly.

bundle2: use compression engines API to obtain decompressor Like the recent change for the compressor side, this too is relatively straightforward. We now store a compression engine on the instance instead of a low-level decompressor. Again, this will allow us to easily transition to different compression engine APIs when they are implemented.

util: remove compressors dict (API) We no longer have any in-tree consumers of this object. Use util.compengines instead.