Performance improvements and backpressure controls.

Simply and speed up command argument processing logic.
Commands now return the true/false value from the underlying socket write(s).
Implement command_queue high water and low water for more better control of queueing.

lib/queue.js

@@ -45,9 +45,13 @@ Queue.prototype.forEach = function (fn, thisv) {
     return array;
 };
 
+Queue.prototype.getLength = function () {
+    return this.head.length - this.offset + this.tail.length;
+};
+    
 Object.defineProperty(Queue.prototype, 'length', {
     get: function () {
-        return this.head.length - this.offset + this.tail.length;
+        return this.getLength();
     }
 });