backend: fix connection won't close if graceful shutdown starts during handshake

[djshow832]

What problem does this PR solve?

Issue Number: ref #161

Problem Summary:

• In server, backend: graceful shutdown #159, if graceful shutdown starts during or before handshaking, the statusConnected or statusHandshaked overrides statusNotifyClose, which makes statusNotifyClose not work.
• When TiProxy receives a SIGINT signal, the cmd.Context() is done, and it will stop some components, such as the goroutine which processes signalGracefulClose. Thus, the BackendConnMgr won't shutdown immediately.

What is changed and how it works:

• Rename connStatus to closeStatus and it only manages the status of closing.
• Create another context in SQLServer. I've checked the code of the gateway, it won't affect the gateway.

Check List

Tests

• Unit test

• Integration test

• Manual test (add detailed scripts or steps below)

• No code

• graceful-shutdown=10 and stop the TiProxy

• graceful-shutdown=10, send a begin statement and stop the TiProxy

• graceful-shutdown=10, send a begin statement, stop the TiProxy, and send a commit statement

• graceful-shutdown=0, send a begin statement and stop the TiProxy

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

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

None

[xhebox]

Rest LGTM