Do not rely on Solr Search Engine when it is avaialble

[gggeek]

There are always chances that the Solr index is slightly out of sync with the db.

For any operations that the are done via migrations, it is thus probably a good idea to always rely on the db-based Search Service.

At the same time: we have to be careful in our usage of a non-standard service (ie. not the one gotten from $repository) - what if we loose user customizations that are required in the Search Service?

NB: this seems a wider-scope rehash of #134 ...
It relates as well to #131 and #207