[RocksDB] Slow getEarliestAvailableBlockSlot

[gfukushima]

While investigating issue #8447 I've found that the nodes running with RocksDB present a significant worse performance on getEarliestAvailableBlockSlot()

Logs for comparison purpose:

Holesky nodes:

LevelDB node: ./teku --config-file=/etc/teku/teku.yml debug-tools db get-earliest-available-block-slot
Time to retrieve earliest available block slot in the database 183ms
1578560

RocksDB node:./teku --config-file=/etc/teku/teku.yml debug-tools db get-earliest-available-block-slot
Time to retrieve earliest available block slot in the database 25398ms
1591424