distsql: check the killed flag in the distsql package

[tiancaiamao]

What problem does this PR solve?

Problem Summary:

Fix an issue from the forum https://asktug.com/t/tidb-new-feature-max-execution-time/309

select user_id,user_name,user_content  from test where user_content = '123456';

There is no index in this query and TiKV scans the whole table.
max_excution_time doesn't work because the killed flag is checked in each Next function,
the code doesn't run to Next until it read sufficient rows in a chunk.

What is changed and how it works?

Check the killed flag in the distsql package level, so the query could break after reading each region result. The code looks like that:

for  {
        if region result == nil {
               fetch next region
        }
        fillChunkWithRegionResult
}

https://github.com/pingcap/tidb/compare/master...tiancaiamao:coprocessor-kill?expand=1#diff-7492d01b7aac81533194642850a45b8eR206

Related changes

#14552
#12852

• Need to cherry-pick to the release branch

Check List

Tests

• No code

Release note

Improve max-execution-time usability by checking the interrupt signal when a new region data is read.

[tiancaiamao]

PTAL @lysu @jackysp

[jackysp]

LGTM

[lysu]

LGTM

[sre-bot]

cherry pick to release-3.0 in PR #15615

[sre-bot]

cherry pick to release-3.1 in PR #15616

[sre-bot]

cherry pick to release-4.0 in PR #15617