re-factoring of Data API (pilot.api.data):   - simplified and fixed workflow for Stage clients   - implemented resolve_replicas() which fetch replica details from Rucio and populates FileSpec objects
re-factoring of Data API (pilot.api.data):
  - simplified and fixed workflow for Stage clients
  - implemented resolve_replicas() which fetch replica details from Rucio and populates FileSpec objects

mlassnig/pilot2,PalNilsson/pilot2,mlassnig/pilot2,PalNilsson/pilot2