Make some function pure virtual for core implementation

This caused problem to have empty implementation with
some compilers.
stream2socket can now handle multiple flux

Using multiple ports will create multiple instance of encoder
[Port] 4242 4343
Will create one instance on port 4242 and one on port 4343

Additionnaly, it is possible to specify cameras to stream:
[Cameras] Cam1 Cam2
Cam1 will be stream to 4242 and Cam2 to 4343

It is still possible to request a specific cam or to go to next cam
on a specific stream