SizedWaitGroup has the same role and API as sync.WaitGroup but it adds
 a limit of the amount of goroutines started concurrently.
 .
 SizedWaitGroup adds the feature of limiting the maximum number of
 concurrently started routines. It could for example be used to start
 multiples routines querying a database but without sending too much
 queries in order to not overload the given database.