Separate KV2 metadata and data keys

[fishi0x01]

metadata and data keys in KV2 are treated equally at the moment.
However, when implementing key/data modification commands such as replace with regexp, this might lead to unexpected and bad behavior.

For that reason, there must be a unified way to separately interact with metadata and data keys in KV2 backends. By default, metadata keys should be ignored by data modification commands.