We used to log a warning when compacting an extra-large partition (CompactionController.getCompactedRow) but took that out as part of
CASSANDRA-6142. Let's add back a warning. (Perhaps in SSTW.append, since MetadataCollector.update doesn't know the partition key and we will want to include that in the log.)
Size threshold should be configurable in cassandra.yaml and default to 100MB.