Make ordinals start at 0.

Our ordinals currently start at 1, like FieldCache did in older Lucene versions.
However, Lucene 4.2 changed it in order to make ordinals start at 0, using -1
as the ordinal for the missing value. We should switch to the same numbering as
Lucene for consistency. This also allows to remove some abstraction on top of
Lucene doc values.

Close #5871