Support interleaving of stream data

[lgrahl]

This makes it possible to interleave messages on different streams. It requires separate buffers for each data channel and a scheduler to handle the pending messages fairly.

[lgrahl]

Reminder that I've added this to Firefox:

#if defined(SCTP_ASSOC_SUPPORTS_INTERLEAVING)
        case SCTP_ASSOC_SUPPORTS_INTERLEAVING:
          LOG(("Supports: NDATA"));
          // TODO: This should probably be set earlier above in 'case SCTP_COMM_UP' but we also
          //       need this for 'SCTP_RESTART'.
          mSendInterleaved = true;
          break;
#endif

[lgrahl]

Reminder that the readme needs updating after resolving this.