Fix build break with C++11

The std::atomics in C++11 don't have a copy constructor so
you need an explicit copy constructor

Change-Id: I833058285c72e4071efa32ddaf12a8f54f7e964f
Reviewed-on: http://review.couchbase.org/34876
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>