Destroy read and write pool connections, once they are drained.

lib/dialects/abstract/connection-manager.js

@@ -44,10 +44,13 @@ ConnectionManager = function(dialect, sequelize) {
 };
 
 ConnectionManager.prototype.onProcessExit = function() {
-  var self = this
+  var self = this;
 
   if (this.pool) {
     this.pool.drain(function() {
+      if (self.pool.read) self.pool.read.destroyAllNow();
+      if (self.pool.write) self.pool.write.destroyAllNow();
+
       self.pool.destroyAllNow();
     });
   }