Replaced ydb_ets_reader with ydb_ets_utils. The main change is that
ydb_ets_utils does not keep track of any state.
- dump_raw/1 and dump_tuples/1 are most useful for monitoring
- apply_diffs/2 is not yet implemented

There are two table formats expected to be in use
(1) Synopses tables: records are of the form {{Op, Timestamp}, ydb_tuple()}
(2) Relation tables: records are of the form {{'row_no', Count}, ydb_tuple()}

Some functions involve specific types of tables:
- combine_partial_results/2 returns a relation table
- add_tuples/3 expects to operate on synopses tables
- all other functions do not care about table format