Can't add c-string and char
Only create a SecureRandom when needed
Return nullstream in don't-log case of MONGO_LOG
SERVER-8244 Pass log levels from MONGO_LOG to Logstream

This is a partial reversion and change of commit
4db60cf23b58a7a5e177e69c142ab9e1503a3b1b.
Use fast thread-local for logging