server: remove etcd

[xhebox]

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

What problem does this PR solve?

Issue Number: ref #136

Problem Summary: Removed etcd. ETCD's HTTP api may conflict with naming of serverless tier

But etcd flags and /api prefix are left. /api prefix will be removed in the next PR for new Handler api.

What is changed and how it works:

1. a btree is used to replace etcd for persistence(in memory only)
2. a refactor on code of config manager to reduce the complexity

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  I hooked a line in LoggerManager.watchCfg

[xhe@PC server]$ echo '{"level":"info","log-file":{"max-size":300,"max-days":1,"max-backups":1}}' | ../../bin/tiproxyctl config log set
2022/11/29 19:13:20.145 +08:00  INFO    main.lgmgr      logger/manager.go:68    updated cfg     {"cfg": {"level":"info","log-file":{"max-size":300,"max-days":1,"max-backups":1}}}

• 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

[djshow832]

Please fix CI.

[djshow832]

Rest LGTM