StoreEngineSpecific to NULL only for specific opcodes (HandleDisconnect)

Context: DEL_VBUCKET, COMPACT_DB

Change-Id: I2316b8c9770d89c3495bf081417e0be4c3dc675f
Reviewed-on: http://review.couchbase.org/39835
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>