MB-12067: Use correct naming style for defragmenter

A few of the defragmenter messages used underscore_seperated_names
instead of camelCase. Fix them.

Change-Id: Ifcce0ac99dfbf1428645cc68993e73f3adf3947c
Reviewed-on: http://review.couchbase.org/44506
Tested-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>