gtid, relay: use gtid_purged to handle gap in Previous_gtids event (#1430)

[ti-srebot]

cherry-pick #1430 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/1441

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

git push [email protected]:ti-srebot/dm.git pr/1441:release-2.0-c32de2423541

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What problem does this PR solve?

part of #1418 (we need to test master-slave upstream manually later)

What is changed and how it works?

remove ResetStart, instead, query @@gtid_purged and combine it to gtid set when needed

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change
• Has interface methods change

Side effects

• Potential performance regression

Related changes

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