Fix warnings in ep-engine

Change-Id: Idc21a4f79b1b927a8e7b14ac2cbf52ac47873229
Reviewed-on: http://review.couchbase.org/30280
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>