BibIndex: common words in virtual index

* Fixes indexing of a virtual index
  in case of removing/modifying a term which is
  in more than one dependent index (closes #1653).

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
WebStat: ingestion health monitor fix

* Removes hardcoded set of indexes to monitor. Takes
  names of indexes to monitor from database instead
  (closes #1631).

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: separate class for virtual index

* Introduces separate class for virtual index.
* Virtual indexes now communicate with their dependent
  indexes through queue table. This approach solves
  problem of parallel execution for virtual index (closes #1661).
* Corresponding fixes for bibindex admin interface and
  regression tests.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: new abstract class for indexes

* Introduces new abstract class for inheriting
  for virtual and regular indexes.
* Cleans WordTable argument list.
* Removes unused imports from bibindex_engine file.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: new DOI tokenizer and index

* Introduces new tokenizer for new
  DOI index (closes #1655).

* Creates new DOI index, field and tag.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>