proxy: move proxyprotocol as a seperate package

[xhebox]

What problem does this PR solve?

Issue Number: ref #245

Problem Summary: Need to support proxy protocol for HTTP, too. Another point is that, epoll refactor will need a much simpler PacketIO and backend structure. So we seperate the package.

What is changed and how it works:

1. move testPipe, testTCP into pkg/testkit
2. seperate proxy protocol
3. add var _ proxyprotocol.AddressWrapper = &originAddr{}

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

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None