Store indexes in Solr

Implement the new IndexProvider SPI to store relevant configured indexes in Solr. This will allow users to define specific indexes and say which (if any) should be stored in Solr, and the query engine would then use them where appropriate (based upon query optimization).

The provider should support indexes for full-text search (full-node and property sets) and normal property types: STRING, LONG, DOUBLE, DATE, DECIMAL, URL, PATH, REFERENCE, WEAKREFERENCE and SIMPLEREFERENCE. There is no need for BINARY properties.