params: refactored sort fields computation.

[alejandromumo]

closes inveniosoftware/invenio-rdm-records#1398

WHY THIS PR:

• I need to implement a type of SortParam, where the fields are computed in the same way as the parent except that, in the new interpreter, the fields are always prepended by a new field.
• To avoid code duplication, I refactored the apply method to just apply the fields to the search's sort instead of computing the fields + applying them.
• Added a service call to reindex records by the latest version first.
  • The idea is to speed up the reindexing of the latest versions while the other versions can be done, in bulk, separately.
• Added a new UOW operation for bulk indexing records, given an iterable of ids (e.g. a generator)