Improve find performance

The goal of this PR is to improve Find performance for exact matching (e.g. contains=false) by using the column type and therefore avoiding unnecessary scan opertions.

For instance:
http://api3.geo.admin.ch/rest/services/api/MapServer/find?layer=ch.bfs.gebaeude_wohnungs_register&searchText=1231643&searchField=egid&returnGeometry=false&contains=false

was taking around 1.5 sec takes now around 0.08 sec.

[Test Link](http://mf-chsdi3.int.bgdi.ch/gal_find_performance/rest/services/api/MapServer/find?layer=ch.bfs.gebaeude_wohnungs_register&searchText=1231643&searchField=egid&returnGeometry=false&contains=false)

@beat-github I hope your customers will be happy ;)

What about adding test(s) for date matching? We could then also test the casting here. (Or are there already date tests?