需求：union all 多时间序列查询

**Is your feature request related to a problem? Please describe.**
比如：
我现在要查一个大的网关设备，下面连着两个子设备，子设备下面有很多测点(1.风速、1.温度、2.湿度)，这两个设备分别上传的数据时间不一样，我想查出的他们的历史的数据，然后带有时间，我查就会出现这样的情况，就像这样

**Describe the solution you'd like**
我希望能够通过类似mysql 中 union all 多表查询，返回一个结果集，比如我可以这样查
select * from root.device.1 slimit 2 align by device union all select * from root.device.2 slimit 1 align by device

**Describe alternatives you've considered**
然后返回结果集可以是这样子的：
root.device.1 -> time,wind_speed,temperature
root.device.2 -> time,humidity

No source files or tests are named in the issue. Start by locating the existing multi-time-series query syntax and its parser or execution tests, then compare the proposed UNION ALL example with current behavior; done means the example returns one result set containing the requested device-specific fields and timestamps.

Written by the indexing model from the issue text.