Filter Encoding : WFS performance issue

[assefa]

This bug is to track the changes related to how the filter encoding is 
interpreted/used according to diffrent underlying layer types.

Right now the OGC Filter Encoding is transformed into Mapserver class 
expression regardless of the underlying layer type. This is very inefficient 
when the underlying data is a database since It does not take advantage of the 
filtering that could be done (Layer's FILTER) on the db before doing a query.