Make multiple observes of the same Mongo cursor share their polling c…

…ycles.

There is no longer any explicit time-based throttling of Mongo poll calls,
though we do prevent multiple instances of polling to be scheduled at once. If
this appears to be a problem, we can reintroduce throttling.

Note: this can lead to a hang if you start observing a cursor during a callback
from an identical cursor's observation. This doesn't seem to be a very realistic
use case though.

packages/livedata/crossbar.js

@@ -20,6 +20,9 @@ _.extend(Meteor._InvalidationCrossbar.prototype, {
   //
   // XXX It should be legal to call fire() from inside a listen()
   // callback?
+  //
+  // Note: the LiveResultsSet constructor assumes that a call to listen() never
+  // yields.
   listen: function (trigger, callback) {
     var self = this;
     var id = self.next_id++;