[FR] readonly mode

Please add a new parameter during database opening to make it only readable,
in case big/expensive database is used or database is on a CD/DVD.

This would also make it possible to open it within multiple processes (LOCK file wont be created).
It would also help with power interruption caused corruption. eg https://groups.google.com/g/leveldb/c/YvszWNio2-Q

Access denied could be returned with write/compaction operation.
I don't mind if leveldb dies because of content changed on disk.