Index datastructure map of docid -> docno

[lgrz]

The feature extraction program extract_features is bound to calling on Indri's metadata. This could be handled by a (docid, docno) map file that is created by Tesserae at index time.

The following could be replaced with a map lookup:

std::vector<docid_t> docids = qry_env.document_ids_from_metadata("docno", docnos);

This also has the benefit of reducing the dependency on Indri to the programs that absolutely require it (i.e the indexing programs).

Related to #13 improving index component.