[FR] readonly mode

[mrx23dot]

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.