Entity Field query got join support

As a new feature the entity Query, the successor of EntityFieldQuery (see http://drupal.org/node/1827278),
got a way to add joins.

There is no active way to define a join, though sub-entities in your conditions will automatically be joined.
Let's take the classical example of listing nodes, with an author with a username starting with a certain string ("cat").

Drupal::entityQuery('node')
  ->condition('uid.entity.name', 'cat', 'STARTS_WITH')
  ->execute();

If you still want to filter by the uid it still works the same:

Drupal::entityQuery('node')
  ->condition('uid', 1)
  ->execute();

Note that this is not yet working because the node entity type is not yet converted to the new Entity system.