Memoize 'location' property of Thing
Revert "Memoize 'location' property of Thing" because it turned out not to make much of a difference in cProfile

This reverts commit 33f439c98691317717af4ca1978b2f3f04c77755.
cache branch and tick, only write them to disk just before commit
caching for Thing and Place and stats thereof
it's more useful with profiling