fixed write pooling release. This makes writes work for master/slave MySQL setups

After 10 writes, the write connection pool would die because connections were not being released.
config:
```js
sequelize = new Sequelize(
  MYSQL_SLAVE.DB
  null
  null
  replication:
    read: [
      host: MYSQL_SLAVE.HOST
      username: MYSQL_SLAVE.USER
      password: MYSQL_SLAVE.PASS
      pool: {} #https://github.com/sequelize/sequelize/pull/1251
    ]
    write:
      host: MYSQL_MASTER.HOST
      username: MYSQL_MASTER.USER
      password: MYSQL_MASTER.PASS
      pool: {} #https://github.com/sequelize/sequelize/pull/1251
)