Prevent permanent waits from destroyed workers

Prevent permanent waits, by ensuring that we continue creating
resources if we need them

 * There are many errors due to pools being configured with min: 0
   but then running out of workers while there is still work to do
 * This results in missed work, infinite loops and timeouts
 * A solution is to ensure that if we still have work todo
   that we make sure we still have some workers to do them

See:

 * brianc/node-pg-pool#48
 * loopbackio/loopback-connector-postgresql#231
 * #175 (Seems related)

lib/Pool.js

@@ -302,11 +302,18 @@ class Pool extends EventEmitter {
    */
   _ensureMinimum () {
     if (this._draining === true) {
-      return
+      return;
+    }
+    const minShortfall = this._config.min - this._count;
+
+    if(minShortfall == 0){
+      const waiting  = this._waitingClientsQueue.size();
+      if(waiting > 0){
+        minShortFall = diff = Math.min(waiting, this._config.max - this._count);
+      }
     }
-    const minShortfall = this._config.min - this._count
     for (let i = 0; i < minShortfall; i++) {
-      this._createResource()
+      this._createResource();
     }
   }