Implement a new dynamic config infra for TiDB Dashboard

[breezewish]

Current defeats:

• All DynamicConfig items share the same notification channel.
• Notification channel does not support external change (for example, performed by CLI).

Proposal new infra:

• Use etcd watch API for notification channel (polling is still needed in case of watch channel is silently broken).
• Store each config item in different keys.

After this new infra is implemented:

• Current KeyViz and AutoProfiling can be migrated to use this infra.
• Some config items in the config file should be moved to use dynamic config, like PublicPathPrefix and EnableExperimental.