Ability to disable KV WAL for all write operations

[valeriansaliou]

WAL would be enabled by default, but could be turned off in the configuration.

This could dramatically improve SSD lifetime on busy servers. Ie. at Crisp's we see a sustained write rate of 2MB/s over all our KV collections, which is fine but could lead to issues over long periods of time.

We would need to force a memtables flush to RocksDB SST files on process shutdown, as to ensure the in-memory log is committed to disk before Sonic goes down.

[valeriansaliou]

We would also need to check if that's needed for batch deletes + atomic deletes.