EZP-21764: Don't instantiate eZINI outside of classes

At the moment, `eZINI` instances for `ezfind.ini`, `solr.ini` and `site.ini` get created outside of the the classes `ezfSolrDocumentFieldBase` and `ezfeZPSolrQueryBuilder`, even when their containing PHP files just get included through the autoloader. This can cause problems, e.g. in the context of Unit Tests when this happens before the INI files get overriden by other processes.

This PR preserves the static class variables, but ensures that they get instantiated inside the classes only.

If have thought about converting them to local variables, but wanted to make sure that derived classes, that access them, don't get problems.

Cheers
:octocat: Jérôme