Fixed dialyzer errors for ydb_ets_utils.
Completed relational SUM aggregate.
- Takes {diffs, Tids} tuples as input.
- Produces [diffs, Tids} tuples as output to listeners.
- Uses ETS table to store intermediate results.

Also added a new function to ydb_ets_utils.
Fixed up ydb_ets_utils after many merge conflicts.