debug for MB-11870

print engine name in dispatcher taskLog
move engine unregistration into WARNING
Change-Id: Ide1206642396f7013017f933a4b5b35cb741c19c
Reviewed-on: http://review.couchbase.org/40239
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>