LOG4J2-455: RingBufferLogEvent should use Messsage timestamp first

[张涛 <ro...@gmail.com>]

https://issues.apache.org/jira/browse/LOG4J2-455

When AsyncLogger received log message (method public void log(final Marker
marker, final String fqcn, final Level level, final Message data,
final Throwable t)), currently it uses clock.currentTimeMillis() (see at
Line 221) as the timestamp.

But it should check the message implements TimestampMessage or not, if yes,
should use TimestampMessage.getTimestamp() as the timestamp, if not , then
using clock.currentTimeMillis().

The new Code:

*data instanceof TimestampMessage ? ((TimestampMessage) data
).getTimestamp() : clock.currentTimeMillis();*