Taxonomy: improvement for querying documents that refer to a category, when deleting it

Following up on HIPPLUG-1023

When deleting categories in the CMS, there is a check for referring documents.

We should make the query (and it's limit) more customizable/configurable to also support cases where the keys are not stored in hippotaxonomy:keys property. E.g. when using a different DAO service [underdocumented] or when having two taxonomy fields [1]

[1] http://taxonomy.forge.onehippo.org/add-extra-taxonomy.html