Introduce association.Listener

[enobufs]

Summary

I found it is difficult to use pion/sctp alone because sctp.Association.Client and sctp.Association.Server take a "connected" UDP. These are equivalent to a child (server) socket and a client socket of TCP. What's missing in pion/sctp s "listening socket".
In the context of WebRTC, we use Client-Client simultaneous open, which is not practical in the cases of non-WebRTC (no-ICE, etc). Client-Server is possible, but it would be difficult to establish connection if there's a NAT in between.

Motivation

Introduce the usability of pion/sctp standalone in TCP style.

Describe alternatives you've considered

I have written a tool that enables TCP like listening socket capability. However, it cannot take advantage of "cookie-echo" (resembles to TCP-Syn-cookie) SCTP offers, which is a measure against DDoS attack (like spoofed SYN-flood in TCP) because internal chunk parsers are not exposed via its API.

We can use go-rudp as a reference.

[enobufs]

Notes:

• This should be addressed along with Turn Stream.WriteSCTP() and Write() a blocking call by default #77
• State Cookie should be modified to follow RFC 4960!
  • Define TCB correctly
  • handleInit() should be decoupled from Association
• Consider bumping major version (to v2)