syncer: update active relay log at fake rotate event (#1448)

[ti-srebot]

cherry-pick #1448 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/1468

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

git push git@github.com:ti-srebot/dm.git pr/1468:release-2.0-1ec8439a21fe

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What problem does this PR solve?

in #1393, we support only specify GTID with no binlog filename, so we can't update active relay log filename at init time. Although active relay log will be updated when after checkpoint-flush-interval seconds and receiving an event, for the first checkpoint-flush-interval seconds, purge-relay will fail because of no active relay log, which may confuse user.

What is changed and how it works?

when only specify GTID with no binlog filename, use fake rotate event to update active relay log.

also, add a integration test to make sure active relay log is forwarded when there's only DML

Check List

Tests

• Integration test

Code changes

• Has exported variable/fields change

Side effects

Related changes

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

[GMHDBJD]

/lgtm

[lichunzhu]

/lgtm

[lichunzhu]

/merge

[ti-srebot]

@lichunzhu Oops! This PR requires at least 1 LGTMs to merge. The current number of LGTM is 0

[lichunzhu]

LGTM