text: Title: Investigate the potential read performance improvement using forward pointers
  Body: Currently the read path of ndarray_store, instead of fetching the segments using a query spec that matches against 'parent' (segment document stores parent as an array of ObjectIDs):
  https://github.com/manahl/arctic/blob/master/arctic/store/_ndarray_store.py#L243
  
  Investigate the potential performance improvements:
  - storing in the version document a list of segment object IDs and query with {'$in': [objID1, objID2....]}
  - storing in the version document a list of segment SHAs and query with {'$in': [sha1, sha2....]}
  
  Investigate using a MongoDB 3.4 or better 3.6 as there have been improvements for the queries using $in:  https://jira.mongodb.org/browse/SERVER-30189
  
  When the prototype solution is developed, benchmark and pay attention to the indexes used, as well as understand the query execution via studying the explain() results.
  
  Test against a variety of scenarios: variable document size, variable number of versions and symbols, variable number of shards, consistency of data.
  
  The solution needs to be backwards compatible.