MB-9173: Refactoring code in IOManager module

Change-Id: Ide088a95a85012e82dad51927d9228b3128a30ff
Reviewed-on: http://review.couchbase.org/29178
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>