Pass server name to after_connect block

I'm currently checking for the host of the server in a `after_connect` callback to apply different settings to it (eg: set a longer statement timeout for a read-only follower).

This is obviously pretty fragile and doesn't work when multiple databases are running on the same host. So proposing here that Sequel also pass the database name to `after_connect`, like:

```ruby
Sequel.connect(URL,
  servers: servers,
  after_connect: proc do |conn, server|
    if server == :default
      # ...
```

By keeping the param order I believe this is backwards compatible.