LuceneDistanceToFieldProjection is not thread safe

LuceneDistanceToFieldProjection mutates its state when contributeCollectors is called, but it can be used simultaneously from multiple threads, so this is not a good idea.

The state should be stored in a separate, thread-specific context object. See what we did in org.hibernate.search.backend.elasticsearch.search.projection.impl.ElasticsearchDistanceToFieldProjection#contributeRequest for example.