obscache: move an assert to a lower level

obscache: use 'nullid' as the hash of an empty obsstore This align this result with what we use for 'emptykey'

obcache: move empty on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obcache: move _checkkey on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obcache: move updateneeded on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obshistory: only display each revision once in debugobshistory When using a revision range, the same precursor could have been displayed more than once. Use a cache to display each revision only once. It's a fix until we found the nicest way to display obs history.

obshistory: display a message when one marker node has no change ctx When exchanging obs markers, there is some change contexts referencing change contexts that are not available locally. As we cannot display informations about them, instead print a message saying so.