In making PluginLogFile thread safe change (If1705b9dd), start method
was still creating AsyncAppender for each thread which is leading to
errors such as "too many open files". This change makes the creation
of AsyncAppender thread safe and only creates one if no AsyncAppender
was created before for a particular log file. Also, acquires a lock
on SystemLog which ensures the thread safety.
Change-Id: I60c2829c28129eb7256ed11f36a77e96cf24f25f
ece0ae9e9b