vlc: load balancing

  Add some initial support for load balancing

  Several VLC instance can be started using the same iptv playlist, This enabls
  concurrent access to "number of VLC instances" channels for clients.

  This commit adds support for this kind of "multi-channel" setup.  When
  multiple VLC instances are available and a client requests playback of a new
  channel, the proxy will attempt to find an available VLC instance in the VLC
  server pool.
  VLC instances are freed when there is no connected client left and put back
  in available state in the server pool.
example: add vlc load balancing sample config