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).