This function should be useful when you need to log during async
parallel operations, without having a mess in logs.
For example when you walk asynchronously a directory and you want logs
to be grouped by file.
It returns the same getLogger() object but messages are stored
internally and sent to appenders only when you call the flush() method
on it.
Updated logger.js to support disabling all log writes.
Updated log4js.js shutdown function to disable log writes.
Added tests.
Update gitignore to ignore rolling date stream's test output.
