auto assign server-id for DM-worker

[csuzhangxc]

Feature Request

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

now, we need to set server-id for DM-worker, but it may conflict with other DM-worker or MySQL/MariaDB slaves.

Describe the feature you'd like:

1. auto assign a non-conflicting server-id
2. set server-id automatically

Teachability, Documentation, Adoption, Migration Strategy:

1. use Server_id in SHOW SLAVE HOSTS to decide whether the server-id already exists in the replication group
2. auto assign non-conflicting server-id (may from special base value)

[csuzhangxc]

#329