*: refine context usage to avoid potential blocks (#1285)

[ti-srebot]

cherry-pick #1285 to release-2.0

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What problem does this PR solve?

refine context usage to avoid potential blocks, like:

fix #1279

What is changed and how it works?

add context for DB operations.

some suggestions for later dev:

• always try to use QueryContext/ExecContext rather than Query/Exec for DB operations
• try to avoid using context.Background for most cases except in testing code
• combine context.Context and logger into a tcontext.Context may be not a good idea, becase context.Context often takes effect in a single process workflow but logger often takes effect in a whole component
• some tcontext.Context may still need to be refined later

Check List

Tests

• Unit test
• Integration test

Code changes

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

Side effects

• Increased code complexity

Related changes

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

[lance6716]

/run-all-tests

[2020-11-23T02:01:29.365Z] go: github.com/mattn/goveralls@v0.0.4: reading http://goproxy.pingcap.net/github.com/mattn/goveralls/@v/v0.0.4.mod: 504 Gateway Time-out

[csuzhangxc]

/run-compatibility-test

[csuzhangxc]

LGTM