MB-16357: Create a variable to get correct locking scope
MB-16357: Create a variable to get correct locking scope

A mistake in 495e00acc24 means that no variable is
created for the ReaderLockHolder, the compiler either
optimises away the lock constructor/destructor or the lock
scope is wrong.

Either way we need to create a variable.

Change-Id: I642ac64d71b73d3d78207ff50d33539a06ce0e7e
Reviewed-on: http://review.couchbase.org/56976
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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