Make the SPATIAL keyword optional when creating spatial indexes. Make it
impossible to build indexes of other types than R-trees on geometry
columns.

This means that geometry columns no longer can have UNIQUE constraints
or be used in primary keys. It is also no longer possible to create
prefix indexes or explicitly ascending or descending indexes on geometry
columns.

Change-Id: Idea1027f11aa278a13442b2237f7ac064ae6b74d