Only keep a single rocksdb debug log file

[5chdn]

Closes #1100

Before:

db $ ls -l LOG*
-rw-r--r-- 1 user users 19001 Aug 21 15:31 LOG.old.1503322205093659
-rw-r--r-- 1 user users 85319 Aug 21 15:30 LOG.old.1503322205071345
-rw-r--r-- 1 user users 88637 Aug 21 15:29 LOG.old.1503322116371839
-rw-r--r-- 1 user users 32292 Aug 21 15:27 LOG.old.1503321102269360
-rw-r--r-- 1 user users 69247 Aug 21 15:32 LOG

After:

db $ ls -l LOG*
-rw-r--r-- 1 user users 67999 Aug 21 15:32 LOG

Rationale:

• There is no documented way to disable the debug/info logging of rocksdb.
• Setting the number of logs to 1 is the cleanest and simplest way to get close to disabling the logs.
• 0 log files is not allowed (out of range error).
• alternatives would involve hacky solutions like piping the logs to /dev/null
• KISS, because we consider replacing rocksdb in the long run