Increase default number of KV-Engine shards from 4 in larger environments.

Given that we can now dynamically adjust the number of writer threads (see MB-22041), the usefulness of this is constrained by the fact that we default to 4 shards - as there's no value increasing the writer threads past the number of shards.

Investigate changing the default number of shards, for example making it based on the number of CPU threads in existence.