*: change user

[xhebox]

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: ref #127

Problem Summary: Minimal effort to support switch auth request when it is change user command. A little bit ugly.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

tiup --db.binpath ./tidb-server
./bin/tiproxy
mvn "-Dcom.mysql.jdbc.testsuite.url=jdbc:mysql://127.0.0.1:6000/test?user=root&password=&useSSL=false&useServerPrepStmts=true" '-Dtest=ConnectionRegressionTest#testBug19354014' test

• No code

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