Improved warm-up of new segments.

* Merged segments are now warmed-up at the end of the merge operation instead
  of _refresh, so that _refresh doesn't pay the price for the warm-up of merged
  segments, which is often higher than flushed segments because of their size.
* Even when no _warmer is registered, some basic warm-up of the segments is
  performed: norms, doc values (_version). This should help a bit people who
  forget to register warmers.
* Eager loading support for field data: when one can't predict what terms will
  be present in the index, it is tempting to use a match_all query in a warmer,
  but in that case, query execution might not be much faster than field data
  loading so having a warmer that only loads field data without running a query
  can be useful.