Have ability to sync empty cohorts

Currently,  a cohort is marked as synched if they have at least one patient belonging to the cohort i.e there must exist a record for a cohort within cohort member in order for it to be marked. There is a need to introduce a flag to track synched cohorts even if they have no patient downloaded eg sync of an empty cohort that will later have patients added to it. A case study of FGH where apps will be setup before patients are assigned to field workers thus the cohort will be having no patient during setup.