pin source bound even the DM-worker instance offline

[csuzhangxc]

Feature Request

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

the load stage has not supported HA now, if a DM-worker instance is offline temporarily, then the source will be scheduled to another DM-worker instance while the new one will lack the full dumped data.

for relay unit, if DM-worker become down because of a temporary problem, its saved relay log will be no longer used and new DM-worker may face the problem that upstream has purged the binlog files.

Describe the feature you'd like:

pin the source bound relationship even the DM-worker instance is offline.

then the user can fix & make the DM-worker instance online again and the DM-worker instance can continue the previous processes.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: