Suppress unsafeNoLock warning

[Wingysam]

I need to disable locking because of an outdated version of lmdb holding the lock on the db I'm trying to use. Everything works properly but only when using readOnly + unsafeNoLock. Please add a second option to suppress the warning.

[wmerfalen]

That particular message is printed directly to stderr from within the C++ code using fprintf. Currently, that would require a custom build to remove those calls which are in src/env.cpp. I would wonder if the author(s) would be okay with a flag that allows the user to specify an option at creation of the lmdb library.