Support dedicated tier

[djshow832]

Development Task

Some modifications to adapt to the dedicated tier:

• server, backend: graceful shutdown #159
• backend: fix connection won't close if graceful shutdown starts during handshake #164
• Retry connecting backends when the backends are not available
• *: adapt to toml #165
• lib/config: reasonable default config values #167
• *: avoid using official atomic.Int64 #169
• fix: adapt to new tiproxy tidb-operator#4821