The background compact thread relies on the session compact API for most of the diagnostic logging. Currently compact will only log info messages by default in MDB when:

• Compact is taking more than 20seconds on a file (compact_progress)
• Compact skips a file because there is no work to do
• Compact times out 

It is useful for the user to know when background compact has compacted a file and how much space it has reclaimed. We should add a info level log messages in wt_background_compact_end when the compact API call has returned to the background compact thread.