Improve statistics / logging from compaction

During investigation of slow background fetches in a customer environment, correlation was found with compaction runs. However currently there's limited insight into what compaction is doing from our log files - for example all we see the start time and end time:

2017-06-28T05:22:23.134793Z NOTICE (bucket) Compaction of db file id: 228 scheduled (awaiting completion).

2017-06-28T05:22:37.969960Z NOTICE (bucket) Compaction of db file id: 228 completed.

It would assist in assessing the impact of compaction if additional details could be logged - for example:

• Size of the input file
• Fragmentation ratio of the input file / size of useful data in the input file.
• Size of the output file
• Number of documents copied into the new file
• Number of documents discarded due to expiration during compaction
• Number of tombstones purged during compaction
• Sequence number which we purged up to.

If the information is already recorded by couchstore then we just need to add it to our logging in kv_engine / couch_compact; if not then further work may be required in couchstore to expose them.