[SPARK-5181] do not print writing WAL log when WAL is disabled

https://issues.apache.org/jira/browse/SPARK-5181

Currently, even the logManager is not created, we still see the log entry
s"Writing to log $record"

a simple fix to make log more accurate

Author: CodingCat <zhunansjtu@gmail.com>

Closes #3985 from CodingCat/SPARK-5181 and squashes the following commits:

0e27dc5 [CodingCat] do not print writing WAL log when WAL is disabled