SingleThreadedRCPtr: Add move constructor from unique_ptr<>
SingleThreadedRCPtr: Add move constructor from unique_ptr<>

Add a move constructor to SingleThreadedRCPtr which allows moving from
a unique_ptr. This improves interopability between our custom
ref-counted ptr and unique_ptr.

Change-Id: Ib0a2caf532ec525260b2713868ed001f5d1f5eae
Reviewed-on: http://review.couchbase.org/75215
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>
Reviewed-by: Manu Dhundi <158873d90a7ef40f3637a222b7329c09d0222554@couchbase.com>

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