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_expect

@@ -0,0 +1,3 @@
+10000
+ERROR:  The requested grid is too big to be rendered
+CONTEXT:  PL/pgSQL function cdb_rectanglegrid(geometry,double precision,double precision,geometry,integer) line 64 at RAISE