syncer: do not ignore fake rotate event to the next file (#1039)

[ti-srebot]

cherry-pick #1039 to release-2.0

---

What problem does this PR solve?

fix #1038

What is changed and how it works?

only skip the fake rotate event if it's not to the next binlog file.

NOTE:

• MySQL server will write a RotateEvent before rotate to the next binlog file normally.
• MySQL server will send a fake RotateEvent to the slave before sending other real binlog events.
• MySQL server will write a StopEvent at the end of a binlog file when stoping itself but without RotateEvent
• MySQL server will write from a new binlog file after started itself

Check List

Tests

• Manual test (add detailed scripts or steps below)
  • trigger the error reported in syncer doesn't rotate to the next binlog file when no RotateEvent in the current binlog file #1038 with an older version of DM
  • replace DM-worker with a newer version in this PR
  • observe the task resumed

Code changes

Side effects

• Increased code complexity

Related changes

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

[lance6716]

LGTM

[csuzhangxc]

LGTM