MB-100 return a reference for operator =
MB-100 return a reference for operator =

For correctness and avoiding unnessary memory copy
overloaded *() operator must return reference instead
of referencing object itself.

Newer versions of gcc (like 4.8) would fails compiling
without the fix.

Change-Id: Id5c747c2a17cff57c6d7d20f567509375677ef32
Reviewed-on: http://review.couchbase.org/26230
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Tested-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>

sriganes/ep-engine,sriganes/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,jimwwalker/ep-engine,daverigby/ep-engine,owendCB/ep-engine,daverigby/ep-engine,membase/ep-engine,hisundar/ep-engine,daverigby/ep-engine,couchbase/ep-engine,daverigby/kv_engine,membase/ep-engine,teligent-ru/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,owendCB/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,hisundar/ep-engine,owendCB/ep-engine,couchbase/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,membase/ep-engine,sriganes/ep-engine,daverigby/ep-engine,daverigby/kv_engine,membase/ep-engine,hisundar/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine