timers: setImmediate process full queue each turn

Previously only one cb per turn of the event loop was processed at a
time, which is not exactly what is meant by immediate

fixes #5798

lib/timers.js

@@ -313,18 +313,24 @@ L.init(immediateQueue);
 
 
 function processImmediate() {
-  var immediate = L.shift(immediateQueue);
+  var queue = immediateQueue;
 
-  if (L.isEmpty(immediateQueue)) {
-    process._needImmediateCallback = false;
-  }
-
-  if (immediate._onImmediate) {
-    if (immediate.domain) immediate.domain.enter();
+  immediateQueue = {};
+  L.init(immediateQueue);
 
+  while (L.isEmpty(queue) === false) {
+    var immediate = L.shift(queue);
+    var domain = immediate.domain;
+    if (domain) domain.enter();
     immediate._onImmediate();
+    if (domain) domain.exit();
+  }
 
-    if (immediate.domain) immediate.domain.exit();
+  // Only round-trip to C++ land if we have to. Calling clearImmediate() on an
+  // immediate that's in |queue| is okay. Worst case is we make a superfluous
+  // call to NeedImmediateCallbackSetter().
+  if (L.isEmpty(immediateQueue)) {
+    process._needImmediateCallback = false;
   }
 }