Major changes to improve performance

* Implement multi-tile caching if cache size is big enough
* Implement multi-tile multi-band caching if cache size is big enough
* Avoid initial full scan table in some situations (no where, PKID index, GIST index)
* Add a AVERAGE_APPROX_RESOLUTION strategy that is now the default, and
  is used to compute quickly the dataset resolution on a small sample of rows,
  when resolution isn't advertized in raster_columns
* Fix caching of connexions in PostGISRasterDriver::GetConnection() to take
  into account database name (and thread)
* Revise how overviews are handled.
* Fix quadtree insertion.
* Fix handling of positive y resolution (ungeoreferenced rasters)
* Optimize retrieval of tiles per PKID (with a map)
* Make driver work when called from non C locale (e.g. QGIS)
* Accept connection strings that have no host=, port= or user= (like OGR PG driver)
* Add tolerance when fetching tiles per coordinates.