Move CiviCRM initialization out of service constructor (in Drupal 8) and into method

Currently, the way to initialize CiviCRM is to construct the service via:

\Drupal::service('civicrm');

However, service constructors really, traditionally shouldn't have any side-effects - the constructor should just get the object ready to be used. Since a service might need to be constructed just to resolve a dependency, it doesn't necessarily mean any work is actually about to be done with the service.

Instead, I think this should be:

\Drupal::service('civicrm')->initialize();

That way you have to explicitly call a method to cause all the side-effects that is initializing CiviCRM.

This is an API change, but since the Drupal 8 integration module isn't officially released, I think now is the time to make it!