gc-compaction: better job split heuristics

[skyzh]

The final stats for downloading is:

INFO compaction_loop{tenant_id=4d77bcd6107af3559d33545edb82b215 shard_id=0308}:run:scheduled_compact_timeline{timeline_id=d245f43c62cd81fc104e168b6786c110}: finish downloading layers, downloaded=14114111488, total=28083863552, ratio=0.50

While the estimation is:

INFO compaction_loop{tenant_id=4d77bcd6107af3559d33545edb82b215 shard_id=0308}:run: splitting compaction job: 000000000000000000000000000000000000..000000067F00004005000080000000D78000, estimated_size=4452556800

4GB vs 14GB

This caused each job is running super slow and spent a lot of time downloading files.