feature: change engine name to tianmu

[hustjieke]

Is your feature request related to a problem? Please describe.

StoneDB is used as a HTAP database name, it is not suitable for used both as an engine name.
As the dissusion result, we choose tianmu as our new eninge name: #199

Describe the solution you'd like

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| cache              |
| mysql              |
| performance_schema |
| sys                |
| sys_tianmu         |
+--------------------+
6 rows in set (0.00 sec)

Describe alternatives you've considered

Additional context