The old implementation searched for all possible values and then
executed each query to see what matches.
The new implementation uses several indices to find only
the matching values.
The reason seems to be the number of memory allocations. In order
to create the union of 100 lists we have 99 memory allocations.
The first needs the space for the first two lists, the second the
space for the first three lists, and so on.
We can reduce the number of allocations drastically (in many
cases to one) by leveraging the fact that many of the lists
were already sorted, non-overlapping and increasing, so that
we can simply concatenate them.
Replaces the use of in-memory data structures with the PersistentMap.
This is the crucial step in reducing memory usage for both persistent
storage and main memory.
Before the offset of the root node was hard-coded.
Now the offset of the pointer to the root node is hard-coded.
That allows us to replace the root node.
