backend, net: track traffic for each client connection

[djshow832]

What problem does this PR solve?

Issue Number: close #175

Problem Summary:
Track traffic for each client (frontend) connection.

What is changed and how it works:

• Track traffic for PacketIO
• Add ClientIO as a member to BackendConnMgr to make track easier
• Add InBytes() and OutBytes for ConnContext

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