EZP-20418: Location Search Handler

This is a reimplementation of #249, taking last comments of @docb22 into account.

* ```Location\Search\Handler::findLocations( $locationCriterion, $offset, $limit )```
  * [x] Implementation in Persistence\InMemory
  * [x] Implementation in Persistence\Legacy (Missing lot of criterion handlers)
* ```Location\Search\Handler::getLocationCount( $locationCriterion )```
  * [x] Implementation in Persistence\InMemory
  * [x] Implementation in Persistence\Legacy
* Refactoring:
  * [x] ```LocationService::loadLocationChildren()```
  * [x] ```LocationService::getLocationChildCount()```