Add specific ID for connections

A new ID is introduced for publishers and subscribers to be able to
identify different connections in the same session. While it was
possible to do that from the Janus handles this ID is meant to be shared
with the clients and set by the peer sending the offer, so it is set by
the client for publishers and by the signaling server for subscribers
(in which case the Janus handle is used).

This will make possible to know without any ambiguity if an offer/answer
belongs to an already existing connection or not. This can be used, for
example, to know if an offer represents an update or a new connection,
or to discard unneeded messages that might be sent in some corner cases
by the clients.

The ID name is "sid" for consistency with the property name used by the
internal signaling server of Talk.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

api_signaling.go

@@ -654,6 +654,7 @@ type AnswerOfferMessage struct {
 	Type     string                 `json:"type"`
 	RoomType string                 `json:"roomType"`
 	Payload  map[string]interface{} `json:"payload"`
+	Sid      string                 `json:"sid,omitempty"`
 	Update   bool                   `json:"update,omitempty"`
 }