lowers logging level for first seen dead compaction

[keith-turner]

For the case of really quick compactions there is a good chance these finish during the check done by the dead compaction detector. Currently when this happens a log message is logged about a possible dead compaction that is a false positive. A large number of quick external compactions can cause a lot of these false positives.

This change adjust the first time a possible dead compaction is logged to trace.

[keith-turner]

I have not had a chance to test this change yet. I made the change by reading the Map.merge() javadoc, would be good to get a double check of the use of Map.merge return value.