BibIndex: edit exactauthor tokenizer

* Renames BibIndexExactAuthorTokenizer in BibIndexSimpleExactAuthorTokenizer.

* Adds new tokenizer BibIndexExactAuthorTokenizer: internally it calls
  BibIndexSimpleExactAuthorTokenizer.

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

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

* Adds new tests for BibIndexSimpleExactAuthorTokenizer.

* Adds test for the new BibIndexExactAuthorTokenizer