Layers not closed when requested

[jerry.pisk@…]

PostGIS layers are not closed when msLayerClose is called which leads to 
memory leaks the next time the layer is used since most mappostgis functions 
do not check for an already open layer (prepare_database for example does not 
check layerinfo->urid_name before allocating new memory for it). The fix will 
include both closing the layer when msLayerClose is called and making sure 
mappostgis does not leak memory.

[jerry.pisk@…]

As far as I can tell this was done before connection pool supported threads. 
No longer needed and since connection pool is now thread safe.

[jerry.pisk@…]

Removed msPOSTGISLayerResultClose, no longer needed. Using connection pool 
even when compiled with threads.