Optimize scheduling cycle if there's no placeholder allocation or reservation

In the scheduling cycle, we call three methods:

• tryPlaceholderAllocate() - tries to replace running placeholders
• tryReservedAllocate() - tries to allocate reservations
• tryAllocate() - tries normal allocation (most common)

Even if we have pending requests, the first two don't always have anything to allocate. In fact, if there's no gang scheduling job, there's no need to run tryPlaceholderAllocate() at all. Similarly, if we don't have reservations, we don't have to call tryReservedAllocate(). Both method walk the hierarchy, copy&filter applications, etc. If we track how many active placeholders and reservations we have, we can eliminate the calls to these two if counter == 0.