Share one UDP socket with all UDP candidates about ice

[enobufs]

Incoming packages should be demux/parsed like this:

[enobufs]

In order to make this possible, we will need to modify pion/stun and pion/turnc to use net.UDPConn which is not bound to a specific remote address, meaning, we should use net.ListenPacket(or net.ListenUDP). NOT net.DialUDP! (it creates a "connected" udp socket!)

Here's a demo

[enobufs]

The new TURN Client (in pion/turn) is almost ready. (see pion/turn#75).

The new client will allow achieving what this PR is aiming for, but let's do it after pion/webrtc#712 successfully lands. (This does not strictly affect connectivity)

Here's what we need to do...
Since currently, all Candidate (concrete) classes share the parent (or embedded in a go's sense) struct, candidateBase which invokes a read-loop (goroutine) on start() - this needs to be changed.

Here's my plan:

• Create a candidateBase per local IP address
• candidataBase also has an instance of turn.Client.
• Let candidateBase creates actuall candidates such as CandidateHost, CandidateServerReflexive, etc. In another word, candidataBase works as a "candidate factory". For instance, candidateBase would have these methods:
  • createCandidateHost()
  • createCandidateSrflx()
  • createCandidatePrflx()
  • createCandidateRelay()
• When createCandidateRelay() is called, candidateBase also allocates a relay port on the TURN server.

[enobufs]

At this point, we will have a proper STUN transaction (and with retransmits) with STUN server, as well as connectivity checking all done by the new turn.Client.