server: add HTTP API to enable/disable async commit and 1PC

[ekexium]

What problem does this PR solve?

Problem Summary:

After upgrading a TiDB cluster, tools like tiup may automatically enable async commit in a cluster.
Tools cannot set global variables directly, so we need to provide an API to do it.

Related issue: tikv/sig-transaction#75

What is changed and how it works?

What's Changed:

Two HTTP APIs were added:

curl -X POST -d "tidb_enable_async_commit=1" http://{TiDBIP}:10080/settings
curl -X POST -d "tidb_enable_async_commit=0" http://{TiDBIP}:10080/settings

curl -X POST -d "tidb_enable_1pc=1" http://{TiDBIP}:10080/settings
curl -X POST -d "tidb_enable_1pc=1" http://{TiDBIP}:10080/settings

Related changes

Check List

Tests

• Unit test

Side effects

Release note

• No release note

[sticnarf]

LGTM

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[youjiali1995]

/LGTM

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[REVIEW NOTIFICATION]

This pull request has been approved by:

• sticnarf
• youjiali1995

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[sticnarf]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 127290d

[ti-chi-bot]

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[sticnarf]

/run-all-tests

[sticnarf]

/run-check-release-note

[sticnarf]

/run-check-title