router: remove connMap

[xhebox]

What problem does this PR solve?

Issue Number: close None

Problem Summary: Instead of keep a map of id <-> conn, we could store additional router data, e.g. pointer to the list element, into conn directly.

What is changed and how it works:

1. Split NewScoreBasedRouter into two functions, another one is Init. It is to cleanup initialization in router_test.go. Gateway may need to adapt it.
2. Add SetValue and Value. It is used to store Elemet[*connWrapper]. We can locate the list element instantly.

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

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None