With the introduction of JDK-8349755, it is not entirely clear any longer that the Async UL has exclusive access to the files which are being logged to in the JVM. We should take the file lock for the stream being logged to for safety's sake.

If, for example, an unattached thread logs, then it will do so without the help of the Async UL thread.

This is expected to very rarely, if ever, happen. Since taking an uncontested mutex is very cheap, I expect this to have a negligible performance impact.