db: investigate optimizations for special keys (locks, intents, txn records)

[RaduBerinde]

In addition to keys containing SQL table data, CRDB transactions read and write other internal keys like locks, intents, txn records. These keys have very special access patterns and can likely benefit from targeted optimizations. They also take up a small fraction of the storage space so we can trade off space for efficiency.

Some ideas of what tweaks we could explore for these keys:

• larger bloom filters, use of bloom filters on L6
• fewer LSM levels
• smaller sstables
• reduced compression

We would need #3957 to be able to break down the work we do for these keys in various workloads.

Long term, it would be ideal if Pebble detected special patterns automatically. But in the short-to-medium term, we can explicitly configure Pebble with specific hints for parts of the keyspace.

Jira issue: PEBBLE-261