Feature: use Solr cores by default

[hannosch]

The Solr developers advertise the use of cores, even for single-index deployments. I think there's good reasons for this and we should follow. Especially being able to build up a new index after a schema change and then transparently swapping it in as the active index without downtime is nice.

This is likely more a documentation task. The buildout recipe currently (3.5) has problems with the core setup. It would also be nice to be able to issue a "@@solr-maintenance/reindex" against a different core.