fix(exex): do not advance backfill range twice

[shekhirin]

this.range.by_ref().take(this.batch_size) in fact advances the original iterator -.-

Will add tests in the subsequent PR, for now this fix works as expected.

Before

2024-09-16T19:56:55.256356Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=0 last=9999
2024-09-16T19:56:55.556711Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=20001 last=30000
2024-09-16T19:56:55.569177Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=40002 last=50001
2024-09-16T19:56:55.578051Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=60003 last=70002

After

2024-09-16T20:00:04.281247Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=0 last=9999
2024-09-16T20:00:04.618367Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=10000 last=19999
2024-09-16T20:00:04.630539Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=20000 last=29999
2024-09-16T20:00:04.648388Z  INFO exex{id="MyExEx"}: alex88_repro: Processing range of blocks first=30000 last=3999