Default value of rocksdb_datadir not ideal

Hi,

I'd like to see a different default value for rocksdb_datadir. The default value of "./.rocksdb" creates a "hidden" directory. This makes supporting the engine unnecessarily complicated. That has these side effects:

• The directory won't show up in ls output of the MariaDB datadir
• The directory won't be included in shell globs
  • rm -r /var/lib/mysql/* won't remove RocksDB files
  • du -sck /var/lib/mysql/* won't show RocksDB files

What is gained by having this directory be "hidden"? Would it be possible to use something like "_rocksdb" instead?