router: add tests for router

[djshow832]

What problem does this PR solve?

Issue Number: close #73

Problem Summary:

• When the sysbench closes, the rebalancer frequently migrates sessions because the connections are unbalanced again and again after closing too many connections.
• Lack of tests.

What is changed and how it works:

• Add a minimal interval to the redirection to avoid migrating too frequently.
• Add a minimal interval for redirection failure to avoid unnecessary retrials. That is, after a connection fails to redirect (maybe because it contains local temporary tables), it won't be redirected again in 3 seconds.
• Add the unhealthy empty backend back to the router when a connection fails to redirect and cannot find the original backend. This avoids wild connections.
• When the balancer searches for a connection to redirect, skip the redirecting connection and search for the next one. This avoids that a connection migrates for too long and blocks other connections from migrating.
• Add tests for the router.

TODO:

• Add more configurations for router #60. I didn't add tests for the redirection interval because it needs to add the interval as a configuration.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Scale-in, scale-out, and close clients when running sysbench.

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None