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 the parent id cache and 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.

Closes #3819