use system_time_zone to encode kv if tidb set it

[3pointer]

What problem does this PR solve?

Resolve #562

What is changed and how it works?

if tidb time_zone is "SYSTEM", try to use "system_time_zone" to set time_zone for lightning session.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Related changes

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

Release note

• Fix the issue that lightning didn't use tidb's time zone to encode timestamp data.

[3pointer]

/run-integration-tests

[glorv]

@3pointer Please add a unit test and an integration test