start
Starts the catch-up for the projection instances selecting them by their identifiers.
If no identifiers is passed (i.e. null) then all of the instances should be caught up
Return
identifier of the catch-up operation
Parameters
since
since when the catch-up should be performed
ids
identifiers of the projections to catch up, or null if all of the instances should be caught up
Throws
if at least one of the selected instances is already catching up at the moment