Add limit to the grid-generating functions

This adds an optinal maxcells parameter to the rectangle and hexagonal grids,
with a default value of 262144 (2^18).
An error occurs when the grids would need more cells.

test/CDB_RectangleTest.sql

@@ -0,0 +1,5 @@
+-- Check small grids are generated...
+SELECT COUNT(*) FROM cartodb.CDB_RectangleGrid(ST_MakeEnvelope(0,0,1000,1000,3857), 10, 10);
+
+-- But large grids produce an error
+SELECT COUNT(*) FROM cartodb.CDB_RectangleGrid(ST_MakeEnvelope(0,0,1000,1000,3857), 1, 1);