obsdiscovery: simply some of the missing computation

discovery: implement some range based discovery This use the previously implemented methods to implement some discovery and fetch method using ranges. There is currently major performance issue and some aspect of the protocol will changes.

discovery: introduce a official 'obshash associated to a range The hash in a combination of the hash a its subrange. If the range is a leaf, we have the obsmarker relevant to the node in that leaf range.

discovery: introduce "stable slicing" methods We introduce new code that leverage the stable sorting to slices a graph in a way "stable" accross repository. This should allow us to use theses slices for obsolescence markers discovery.

discovery: adds a function doing revnum independant sorting The function is doing a topological sort without depending on revision number. This make the sort independent of the repository and suitable for discovery.

hgignore: also ignore the 'dist' directory This is generated by the 'sdist' command

obsexchange: drop unused module shame shame shame

MANIFEST.in: also includes topic That was also overlooked when we added topic.