Global role computation is very expensive, don't do it when all we are
interested in is user existence.
Also, list all users having local roles in one call to reduce the effect
of per-call overhead.
This should improve indexation speed by a large fraction: pprofile
statistical run on and activity node busy reindexing objects shows:
- 57% of time spend generating (13%) and executing (44%) SQL
- 42% of time spent preparing object for indexation:
  - 29% in _getSecurityParameterList, all in getUserById
  - 12% in getCatalogVariablesFor (workflow variables to index)
So an improvement of close to 30% is expected.