Closing the ssh tunnel

[aslakhellesoy]

Hi - thanks for this great library!

I've been using ssh2 to create a tunnel as described in the README.

I found that the SSH connection hangs unless it's explicitly closed when the postgres connection is closed:

import ssh2 from 'ssh2'

const ssh = new ssh2.Client()

const sql = postgres({
  ...options,
  socket: ({ host: [host], port: [port] }) => new Promise((resolve, reject) => {
    ssh
    .on('error', reject)
    .on('ready', () =>
      ssh.forwardOut('127.0.0.1', 12345, host, port,
        (err, socket) => err ? reject(err) : resolve(socket)
      )
    )
    .connect(sshOptions)
  })
})

const oldEnd = sql.end.bind(sql);
sql.end = async () => {
  ssh.end();
  await oldEnd();
};

If you agree with this approach, I'd be happy to send a PR to add this to the docs.