Proxy: Support multiple origin servers for a stream.

[winlinvip]

This feature is based on the Proxy server for SRS.

What is the business background? Please provide a description.
For stream backup, a stream can exist on two origin servers, so when one stream is down, the proxy can switch to the other.

If multiple stream backups are supported, meaning a stream can be published to multiple origin servers, then the proxy can support more players and is not limited to a single origin server.

Is your feature request related to a problem? Please describe.
Nop.

Describe the solution you'd like
For the proxy, it selects an origin server when publishing or playing a stream, so it only supports one origin server per stream. Need to update the proxy server: when publishing a stream and the stream already exists, it should select a new origin server. When playing, if there is more than one origin server hosting this stream, select an origin server randomly.

Describe alternatives you've considered
Nop.

Additional context
If a user publishes a stream directly to the origin server, the proxy server currently does not detect it. The proxy should add an HTTP API to accept SRS HTTP callbacks about publish events and update the origin servers accordingly.