i.e. allow minetestmapper to query the database for a list of
blocks in a specific range (corresponding to the requested geometry),
instead of always obtaining a full list off all blocks.
As postgresql is the only database which supports this efficiently,
this option is only effective for postgresql.
As tests using --sqlite-cacheworldrow showed a consistently
lower performance than without, this option was effectively
disabled. It is still recognised for compatibility, but
it may be removed some time in the future.
