worker/: fix worker stuck in unitTransWaitCondition(#589)

[lichunzhu]

cherry-pick #589

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What problem does this PR solve?

#576
When subtask is started, it will use unitTransWaitCondition to wait for relay log to catch up. But if this task is paused or stopped, we should quit as fast as we can instead of keeping waiting.

What is changed and how it works?

1. Use st.currCtx to control wait of unitTransWaitCondition.
2. Use parent context st.ctx and son context st.currCtx to make sure task can be shut down.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

[lichunzhu]

/run-all-tests

[codecov]

Codecov Report

Merging #622 into release-1.0 will increase coverage by 0.1757%.
The diff coverage is 89.6000%.

@@                 Coverage Diff                 @@
##           release-1.0       #622        +/-   ##
===================================================
+ Coverage      57.6743%   57.8500%   +0.1757%     
===================================================
  Files              166        166                
  Lines            16933      16968        +35     
===================================================
+ Hits              9766       9816        +50     
+ Misses            6219       6203        -16     
- Partials           948        949         +1     

[WangXiangUSTC]

LGTM

[csuzhangxc]

LGTM