NCBC-457: Change verbosity of HB logging to DEBUG

Change-Id: Iddada42d7adffeebffc86321893fe9b09a9640f0
Reviewed-on: http://review.couchbase.org/36715
Reviewed-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>