MB-20330: ReaderLockHolder with no lvalue
MB-20330: ReaderLockHolder with no lvalue

3.x merge brought in the wrong version of some
code meaning that a read lock is never acquired.

Change-Id: I139ac041d54fdf8d459f4309a9c2be22e40afb8e
Reviewed-on: http://review.couchbase.org/66282
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Will Gardner <cbd6f5d9092d9ef7d5c0510d53bba134b22c02f2@couchbase.com>
Well-Formed: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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