syncer: fix ignore canceled error

[csuzhangxc]

What problem does this PR solve?

in #355, ignore canceled errors for executing SQLs is introduced.

but miss to ignore the canceled errors for executing DMLs.

What is changed and how it works?

check !utils.IsContextCanceledError(err) before s.runFatalChan <- unit.NewProcessError(errType, err) for executing DMLs as we did for DDL.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  • hack the code for executing DMLs and return an error in one goroutine
  • obverse the log and result of query-status for context canceled error

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note (with loader/syncer: filter context cancel error while executing sqls #355)

[csuzhangxc]

/run-all-tests tidb=release-3.0

[csuzhangxc]

@WangXiangUSTC @amyangfei PTAL

[codecov]

Codecov Report

Merging #401 into master will increase coverage by 0.1258%.
The diff coverage is 0%.

@@               Coverage Diff                @@
##             master       #401        +/-   ##
================================================
+ Coverage   57.5617%   57.6876%   +0.1258%     
================================================
  Files           163        163                
  Lines         16471      16442        -29     
================================================
+ Hits           9481       9485         +4     
+ Misses         6065       6036        -29     
+ Partials        925        921         -4

[WangXiangUSTC]

LGTM

[amyangfei]

LGTM

[sre-bot]

cherry pick to release-1.0 in PR #402