Log a message when rolling a thread back because it holds the oldest timestamp

There is some code in WiredTiger that automatically rolls back the oldest transaction that's done any updates, if the cache gets stuck full:

https://github.com/wiredtiger/wiredtiger/blob/develop/src/evict/evict_lru.c#L2306

It is an unusual path, and logging something to the message event handler would make diagnosing such a failure mode easier. If logging a message in that case makes internal testing noisier we could consider adding a new verbose=rollback_messages or similar configuration string.