Store _version as a numeric doc values field.

Doc values can be expected to be more compact than payloads and should provide
better flexibility since doc values formats can be picked on a per-field basis.
This patch:
 - makes _version stored as a numeric doc values field,
 - manages backwards compatibility: if a version is not found in doc values,
   then it will look into paylods,
 - uses background merges to upgrade old segments and move _version from
   payloads to doc values.