Cleanup: topkeys is a std::unique_ptr
Cleanup: topkeys is a std::unique_ptr

(Use the bool operator instead of compare with != nullptr)

Change-Id: Ifb149633746e07428d204a8904dc873eb1c00a8f
Reviewed-on: http://review.couchbase.org/119404
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine