router: refine BackendObserver and add tests

[djshow832]

What problem does this PR solve?

Issue Number: close #58

Problem Summary:

• The status port cannot stand for the real status of the SQL server.
• Lack of tests

What is changed and how it works:

• Also fetch /topology/tidb/ip:port/ttl to filter tombstone backends
• Dial both the status port and the MySQL protocol port to check the health
• Add tests

I also proposed 2 enhancements for TiDB graceful shutdown: pingcap/tidb#37471

TODO:

• Expose some configurations, such as health check interval, just like HAProxy Add more configurations for router #60
• Split BackendStatus to a separate file
• Add TLS options to the HTTP status and etcd client connections

Check List

Tests

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

Scale out and scale in the TiDB cluster during 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

Please refer to Release Notes Language Style Guide to write a quality release note.

None