Currently we choose BE random without check disk is available,
the create table will failed until create tablet task is sent to BE
and BE will check is there has available capacity to create tablet.
So check backend disk available by storage medium will reduce unnecessary RPC call.
* Support bitmap_intersect
Support aggregate function Bitmap Intersect, it is mainly used to take intersection of grouped data.
The function 'bitmap_intersect(expr)' calculates the intersection of bitmap columns and returns a bitmap object.
The defination is following:
FunctionName: bitmap_intersect,
InputType: bitmap,
OutputType: bitmap
The scenario is as follows:
Query which users satisfy the three tags a, b, and c at the same time.
```
select bitmap_to_string(bitmap_intersect(user_id)) from
(
select bitmap_union(user_id) user_id from bitmap_intersect_test
where tag in ('a', 'b', 'c')
group by tag
) a
```
Closed#3552.
* Add docs of bitmap_union and bitmap_intersect
* Support null of bitmap_intersect
