This data structure contains an adjacency list of
backfill nodes.
Each edge represents a backfill order.
For instance, given:
BackfillOrders[1] = [2, 3, 4]
It means that node 1 must be backfilled before nodes 2, 3, and 4.
Concretely, these node ids are the fragment ids.
This is because each fragment will only have 1 stream scan,
and stream scan corresponds to a backfill node.