balance, proxy: support evicting backends by config (#1116)

[ti-chi-bot]

This is an automated cherry-pick of #1116

What problem does this PR solve?

Issue Number: close #1115

Problem Summary:
When a backend is confirmed to fail, we need a way to evict it manually.

What is changed and how it works:

• add proxy.fail-backend-list and proxy.failover-timeout config
• stop routing new connections to failed backends and migrate existing ones away
• force close remaining connections after the failover timeout
• if no more available backends, ignore the list to be HA
• allow fail-backend-list to match by pod name or backend address

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

- Support evict failed backends by config

[ti-chi-bot]

@djshow832 This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be LGTMed and approved by the reviewers firstly.
2. For pull requests to TiDB-x branches, it must have no failed tests.
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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-build	5550fc1	link	true	/test pull-build
pull-check	5550fc1	link	true	/test pull-check
pull-mysql-connector-test	5550fc1	link	true	/test mysql-connector-test
pull-unit-test	5550fc1	link	true	/test pull-unit-test

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