net: fix proxy protocol

[xhebox]

What problem does this PR solve?

Issue Number: close #273

Problem Summary: tiproxy did not try to parse PP headers. PP is disabled by default during introducing serverless support. Since then it is always disabled, and never get fixed.

What is changed and how it works:

1. WithProxy should set proxyInited = false.
2. Add unit tests to prevent regression.

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.

None