Fixes "InvalidParameterException: Log events in a single PutLogEvents request must be in chronological order." and also a race condition

[almson]

List of events needs to be sorted by timestamp before sending to CloudWatch.

Base class AppenderBase already uses synchronized methods, so it's not a problem to make other methods synchronized. This simplifies concurrency issues. Previously, there was a bug on line 71 of AwsLogAppender.