The action REBUILD causes the indexer thread to rebuild the index from
scratch, re-fetching all entities This should only ever appear on the
master record
At the moment this is a placeholder, but eventually
It will return a block of Custom RDF, that the EntityContentProducer wants
the search index to index.
This is the same as standard search, but the caller can specify, by name, the
index Filter and the index Sorter by name
The Sorter and the Filter will be consulted during the search, and hence should not
make massive demands on the framework, otherwise they will cripple the search
performance