backend: use BackendConnMgr as ConnContext

[xhebox]

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

What problem does this PR solve?

Issue Number: close #174

Problem Summary: A cleanup/refactor PR for backend, proposed when thinking on how to implement compression protocol elegantly(most logic will be in Authenticator). Check #174.

What is changed and how it works:

1. Move ctxmap and clientAddr, the core of ConnContext API into BackendConnMgr
2. Remove supportedServerCapabilities, replaced by handshakeHandler.GetCapability()
3. WriteInitialHandshake(), readInitialHandshake() and ParseInitialHandshake() now use pnet.Capability instead of uint32
4. Refactor writeAuthHandshake and share common code in handshakeFirstTime and handshakeSecondTime
5. Add WithRemoteAddr() to *PacketIO. Now PacketIO.RemoteAddr() will return the given remote addr if any.
6. Remove clientIO.SourceAddr(), which is solely used by proxy protocol. It is now replaced by the new WithRemoteAddr()+RemoteAddr() comb.
7. Remove BackendConn, which only has one useful method Addr(), replaced by the new WithRemoteAddr()+RemoteAddr().
8. Remove auth.serverAddr, which is used by TLS handshake, replaced by the new WithRemoteAddr()+RemoteAddr().

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 (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]

While I do want to keep the test, the logic of ClientAddr() moves to BackendConnMgr instead of Authenticator. Also, this test will fail on linux without IPV4. So I just removed it, maybe test BackendConnMgr more later.