支持语法create database if not exists

*Issue migrated from trac ticket # 897 [www.kunlunbase.com](url) *

**component:** computing nodes | **priority:** major

#### 2022-07-12 13:38:53: smith created the issue


___
> ```
> abc=# create database if not exists abc;
> WARNING:  database "abc" already exists
> CREATE DATABASE
> abc=#
> ```