Allow configuring max connections in the app config.

Hi, I found it useful to have a method to override max_connections from the app config. On heroku the database config file is autogenerated; in principle it's possible to configure max_connections in a query option on database url, but this is something which is easy to forget and the connection pool size is something rather app-specific anyway.

The patch makes it possible to override max connections in the application configuration, so eg. if you put 
```ruby
  config.sequel.max_connections = 16
```
in config/environments/production.rb you get connection pool size 16 in production environment.

See also the discussion at https://github.com/jeremyevans/sequel/issues/638