intermediate base indexers for reducing code redundancy

[tracyhenry]

from Kevin: Each of the subclasses of index.java have a lot of the same code. If we broke up the steps in createMV roughly into the commented steps and then the meat inside the main loop in a more functional style it might make it more clear to future implementers what little needs to be changed. I would also not be afraid to create a middle level of class if two or more subclasses have a lot in common. This might help also to create a common base for both PostgreSQL and MySQL flavors of intermediate classes for Spatial, Tile, etc.

wenbo: Not every indexer shares the same code flow, but basic ones (psql, mysql) do have a lot of in common and can be abstracted to be more clear.