findOne优化 (#1772)

1、直接调用runCommand方法
2、增加"limit": 1参数，限制只返回一条数据

lualib/skynet/db/mongo.lua

@@ -494,11 +494,12 @@ function mongo_collection:raw_safe_delete(delete)
 end
 
 function mongo_collection:findOne(query, projection)
-	local cursor = self:find(query, projection)
-	if cursor:hasNext() then
-		return cursor:next()
+	local r = self.database:runCommand("find", self.name, "filter", query and bson_encode(query) or empty_bson,
+		"limit", 1, "projection", projection and bson_encode(projection) or empty_bson)
+	if r.ok ~= 1 then
+		error(r.errmsg or "Reply from mongod error")
 	end
-	return nil
+	return r.cursor.firstBatch[1]
 end
 
 function mongo_collection:find(query, projection)