pkg/lightning: replace local generated ts with tso fetch from pd

[glorv]

What problem does this PR solve?

Replace commit ts generated with local time by tso fetch from pd. This can address the issue that if local time is mush faster than tidb cluster time, after import data with local/importer backend, query may returns empty result.

What is changed and how it works?

• Fetch a ts from pd and store it in RestoreController and use this ts for all WriteRows
• Always fetch the newest ts from pd when do table checksum. This is useful when we support concurrent import with many lightnings and only do checksum after the last lightning finished.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• Fix the bug that lightning generated ts may be to large or small that query may return incorrect result.

[kennytm]

rest LGTM for now.

i don't particularly like requiring TS for TiDB backend, but let's refactor this later.

[kennytm]

/lgtm

[glorv]

@lance6716 @Little-Wallace PTAL

[lance6716]

/lgtm

[glorv]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

cherry pick to release-4.0 in PR #860