Limit maxmind local subselect query to reduce cost

Since we only want to select one IP block anyway, limiting this query
reduces the query cost from (cost=22531.65..22539.67 rows=1 width=31)
to (cost=0.53..8.54 rows=1 width=31)
Merge pull request #726 from toothrot/master

Limit maxmind local subselect query to reduce cost