patch pool.js to be async consistently

Author: Anton Lakin

lib/Pool.js

@@ -15,7 +15,9 @@ function Pool(options) {
 
 Pool.prototype.getConnection = function (cb) {
   if (this._closed) {
-    return cb(new Error('Pool is closed.'));
+    process.nextTick(function(){
+      return cb(new Error('Pool is closed.'));
+    });
   }
 
   var connection;
@@ -46,7 +48,9 @@ Pool.prototype.getConnection = function (cb) {
   }
 
   if (!this.config.waitForConnections) {
-    return cb(new Error('No connections available.'));
+    process.nextTick(function(){
+      return cb(new Error('No connections available.'));
+    });
   }
 
   this._connectionQueue.push(cb);