readme: add a changelog entry about the more efficient obscache

obscache: extract a data agnostic class We now have an independent class that we can reuse for other purpose (eg: obshashrange cache)

obscache: extract the actual data update in a dedicated function This will help extract a data agnostic class.

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.