Parse attribute section ssrcs semantically

[ekr]

///////////////////////////////////////////////////////////////////////////
// a=ssrc, RFC5576
//-------------------------------------------------------------------------
// ssrc-attr = "ssrc:" ssrc-id SP attribute
// ; The base definition of "attribute" is in RFC 4566.
// ; (It is the content of "a=" lines.)
//
// ssrc-id = integer ; 0 .. 2**32 - 1
//-------------------------------------------------------------------------
// TODO -- In the future, it might be nice if we ran a parse on the
// attribute section of this so that we could interpret it semantically.
// For WebRTC, the key use case for a=ssrc is assocaiting SSRCs with
// media sections, and we're not really going to care about the attribute
// itself. So we're just going to store it as a string for the time being.
//