Support MCU for WebRTC, or interactive live streaming.

[winlinvip]

MCU is required for some embeded device.

ChromeA --WebRTC--->---+        +------------+
ChromeB --WebRTC--->---+-->-----+  SRS(MCU)  +---WebRTC-->-- Embeded Device
ChromeC --WebRTC--->---+        +------------+

Note: Chrome A/B/C use SFU to interact with each other, which is supported by SRS 4.0. For some embeded device, it only supports one stream, so SRS should support MCU to merge multiple streams to one, then deliver to it.

For interactive live streaming, also need MCU to merge two live stream.

ChromeA --WebRTC--->---+        +------------+
ChromeB --WebRTC--->---+-->-----+  SRS(MCU)  +---RTMP-->-- Live
ChromeC --WebRTC--->---+        +------------+

[winlinvip]

We will try solution #3459 first, if it works, we won't do it in SRS.