worker, ha: increase keepalive TTL to 1 minute, and to 30 minutes if relay enabled (#1405)

[ti-srebot]

cherry-pick #1405 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1426

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/dm.git pr/1426:release-2.0-50876d3d597a

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What problem does this PR solve?

make keepalive more rubost

What is changed and how it works?

after this PR there're two keepalive TTL: one for relay and one for non-relay. They are 30 minutes and 1 minute seperately by default.
If relay task is assigned, change it to relay-keepalive-ttl, when no relay tasks, chage it to keepalive-ttl

Check List

Tests

• Integration test

Code changes

• Has exported function/method change

Side effects

• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note