auto detect upstream server type (flavor)

[csuzhangxc]

Feature Request

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

now, we need to set flavor for DM-worker, but it may set incorrectly.

Describe the feature you'd like:

1. detect upstream MySQL/MariaDB server type
2. set flavor automatically if detected

Teachability, Documentation, Adoption, Migration Strategy:

use select @@version or other variables to detect server type

[csuzhangxc]

done in #323