Switch to query-time boosting to support Solr 7 and better

[znerol]

Solr 7 dropped support for index-time boosting. From the changelog:

Index-time boosts have been removed from Lucene, and are no longer available from Solr. If any boosts are provided, they will be ignored by the indexing chain. As a replacement, index-time scoring factors should be indexed in a separate field and combined with the query score using a function query. See the section Function Queries for more information.

Let's use query time boosting instead (and maybe make that configurable in the plugin settings in a second PR).