BibIndex: exactauthor tokenizer for parent records

* Adds tokenizer BibIndexExactAuthorParentTokenizer: internally it calls
  BibIndexExactAuthorTokenizer.

* Hardcode in bibindex_engine that the 'exactauthor' field tokenizer wants
  a record id insthead of a tag value.

* Sets BibIndexExactAuthorParentTokenizer as tokenizer for the
  'exactauthor' index (tabfill.sql and upgrade script)

* Adds tests for BibIndexExactAuthorParentTokenizer and
  BibIndexExactAuthorTokenizer