implement aggregate_locking design

implement aggregate_locking design

Now aggregation will not lock the wiki. Any changes made during aggregaton are
merged in with the changed state accumulated while aggregating. A separate
lock file prevents multiple concurrent aggregators. Garbage collection
of orphaned guids is much improved. loadstate() is only called once
per process, so tricky support for reloading wiki state is not needed.

(Tested fairly thuroughly.)