storage: investigate more aggressively sizing block cache

In sysbench oltp_read_write (10x10m rows), we see ~0.5% of cpu time in syscalls reading SSTs, presumably due to block cache misses.
The dataset at this point is ~39gb per node (`df -h /mnt/data1`), which is not much larger than available memory. At the same time, actual mem usage on these nodes is just around 30-40%.

It stands to reason that performance could improve with a larger block cache. We should validate this and if so understand why we're not using more memory on this workload out of the box.

Jira issue: CRDB-43618