Use estimated metadata for PostGIS layers

[jpalmer]

PostGIS layers with millions of features take quite some time to be added to the map canvas because of table feature counts, layer extents and geometry type determination.

Attached is a patch that adds an option to speed up the following PostGIS layer operations:

• The row count is determined from table statistics obtained from running the PostgreSQL table analyse function.

• Table extents are always determined with the estimated_extent PostGIS function, even if a layer filter is applied.

• If the table geometry type is unknown and is not exclusively taken from the geometry_columns table, then it is determined from the first 100 non-null geometry rows in the table. This applies to both the PostGIS Provider and the Add PostGIS Layers dialog.

This patch potential resolves the following tickets: #2394, #581

[jef]

slightly modified applied in r13044. Thanks Jeremy.

[jef]

take 2 on r13221.