readme: rearrange changelog content blackbox is more important than the various caches changes.

obshistory: rename the command to "olog" The command start to look good enough to be handed out to user. We replace the old and useless "olog" alias in favor of the new command.

obshistory: move the command into the obshistory module The root module is far too large already.

obshistory: add docstring to cmddebugobshistory Add debugobshistory command documentation based on the log command documentation.

obshashrangecache: make sure we re-warm the cache after a reset This will "mitigate" the effect of dropping existing entries when new markers affect existing range.

obshashrangecache: precisely track affected revs when adding new markers This will be useful when we start doing finer grained invalidation (instead of dropping all data).