Removed the generated flag from Region. This flag was only being used by
the WorldGenerator for thread regulation purposes, and exposing it to
other objects risked concurrency problems in that a rogue thread could
trick multiple threads into generating the same region simultaneously.
Now, the WorldGenerator maintains mappings of regions (technically,
their keys) to equivalent flags, interactions of which are privatised to
WorldGenerator to ensure safety.
Also, I realised the list being used in Log didn't need to be
synchronised.