remove reference to pool when destroying

Leaving a reference to the pool means the
destroyed pool will get returned if you
later try to reconnect with the same
connection string.

Author: aaronyo

Diff for: lib/index.js

@@ -22,6 +22,7 @@ PG.prototype.end = function() {
   var self = this;
   Object.keys(self.pools.all).forEach(function(key) {
     var pool = self.pools.all[key];
+    delete self.pools.all[key];
     pool.drain(function() {
       pool.destroyAllNow();
     });