ST_ClusterDBSCAN shouldn't join clusters by their borders

[dbaston]

In the DBSCAN algorithm, elements of a cluster are either

• "core geometries", which are within eps distance of minPoints other geometries, or
• "border geometries", which are within eps distance of a core geometry

This leads to a situation where a (border) geometry g may not be within eps distance of minPoints other geometries, but is within eps distance of core geometries in two or more different clusters A, B, …. etc. In this case, rather than merging clusters A and B into a single cluster, g should be assigned arbitrarily but deterministically to either A or B, which should remain separate.

[dbaston]

Applied changes to trunk at r14934