Generate extent for features assigned to default spatial context

[mloskot]

If a feature class (table) belongs to default spatial context (SRID = -1) then spatial extent is also set according to default extent with min values set to -10000000 and max values set to 10000000.

This is not optimal for clients so it's highly suggested to assign real extent to such feature classes using default spatial context.

For better performance, spatial extent is estimated during maintenance operation which update query planner statistics, so users will be required to run VACUUM or ANALYZE frequently. Otherwise, spatial extent is set Zeros or big-default-values.

[mloskot]

Patch submitted for review as #318

[mloskot]

Patch has been applied to the trunk (r3904). I'm closing it as fixed. Please, reopen if something is still not working well.