Converge load_existing_pivot_data() and load_pq_pivots_bin()

[arrayka]

It seems that we have two functions doing the same thing - loading pq pivots:

1. load_existing_pivot_data
2. load_pq_pivots_bin

We should converge them (in a separate PR). Let me create a work item for that.

Originally posted by @arrayka in #1010 (comment)