MB-8798 Remove dead conns by iterating the conn list one time.

Change-Id: I04a9e64462391b7b93249828d1ecbef3dfd60175
Reviewed-on: http://review.couchbase.org/28107
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>