Reduce memory usage in ST_ClusterWithin / ST_ClusterDBSCAN

[dbaston]

The make_strtree function in lwgeom_geos_cluster.c prepares a GEOS STRTree of LWGEOM pointers. To store the LWGEOMs in the STRTree, it has to provide GEOSSTRtree_insert with a GEOS geometry whose envelope corresponds to the envelope of the LWGEOM. It's currently doing this with GBOX2GEOS, which produces a rectangular GEOS Polygon. Since this geometry is never used directly — it's basically a container for an envelope — this should be replaced with either a GEOS Point or a two-point GEOS LineString, depending on the LWGEOM that's being stored in the tree.

This will reduce a significant memory overhead, particularly when clustering functions are used on point data sets.

[dbaston]

Committed to trunk at r14979. When clustering points, these changes reduce peak memory usage by ~45%. Run time is also improved.