Use Agent._src getter for presence src

This change updates the Presence `src` field to use the Agent's `_src`
getter, which will try to use a client-proposed ID, and then fall back
to the server-generated `clientId`.

This means that the `src` should be consistent across client
reconnections, which helps `RemotePresence` to determine which ops have
come from which presence providers.

Author: Alec Gibson

lib/agent.js

@@ -753,7 +753,7 @@ Agent.prototype._createPresence = function(request) {
   return {
     a: 'p',
     ch: request.ch,
-    src: this.clientId,
+    src: this._src(),
     id: request.id, // Presence ID, not Doc ID (which is 'd')
     p: request.p,
     pv: request.pv,
@@ -801,7 +801,7 @@ Agent.prototype._requestPresence = function(channel, callback) {
 };
 
 Agent.prototype._handlePresenceData = function(presence) {
-  if (presence.src === this.clientId) return;
+  if (presence.src === this._src()) return;
 
   if (presence.r) return this.send({a: 'pr', ch: presence.ch});