Expert: attributes streaming definition for indexing facets

Steaming of facets attributes is a low level indexing interface with Lucene indexing. There are two types of category related streams:

• Category tokenizer stream handles tokenization for a single category, e.g. for creating drill-down tokens.
• Category list tokenizer stream handles tokenization for multiple categories, e.g. for creating a counting list token, representing all the categories of a certain document.