Support multiple passphrase by multiple srt_server

[winlinvip]

To support multiple passphrase for SRT.

It's impossible to support different passphrase for a srt_server, because passphrase is set on transport or srt_server, so you should use http callback if use one srt_server.

But, if support multitple srt_server, it's possible to support multiple passphrase:

srt_server {
    listen 10080;
    method publish; # The default m is publish
    passphrase xxx; # The passphrase for publish
}

srt_server {
    listen 10081;
    method request; # The default m is request
    passphrase yyy; # The passphrase for request
}

The multiple srt_server is useful not only for passphrase, bu it also enable user to support default method by port, for example, 10080 is publish, and 10081 is request, which is essential if device doesn't support SRT URL.

Note that the env config can only be applied to the first srt_server.