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

@@ -304,7 +304,14 @@ class Pool extends EventEmitter {
     if (this._draining === true) {
       return
     }
-    const minShortfall = this._config.min - this._count
+    let minShortfall = this._config.min - this._count
+
+    if (minShortfall <= 0) {
+      const waiting = this._waitingClientsQueue.length
+      if (waiting > 0) {
+        minShortfall = Math.min(waiting, this._config.max - this._count)
+      }
+    }
     for (let i = 0; i < minShortfall; i++) {
       this._createResource()
     }